Fix `-Wsfinae-incomplete` warnings when building with GCC 16.x #949

According to the CMake documentation for AUTOMOC, all moc output files that are not included in a source file are aggregated into the CMake-generated <AUTOGEN_BUILD_DIR>/mocs_compilation.cpp, which is added to the target's sources.

Within that single translation unit, moc-generated code checks the completeness of a signal or slot parameter type while it is still only forward-declared, and the type is completed later, when a subsequently included moc_*.cpp file pulls in the header that defines it. GCC 16.x diagnoses this pattern with the -Wsfinae-incomplete warning, which is enabled by default.

Including the moc output files at the end of the corresponding source files excludes them from mocs_compilation.cpp, so each one is compiled in a translation unit where the relevant types are complete.

FWIW, Qt itself uses the same approach throughout its own codebase. Also see https://www.youtube.com/watch?v=Cx_m-qVnEjo.

Steps to reproduce on the master branch @ 18c05d93016b28a9afd4c716dfe00b6e0accb30b on Fedora 44 (GCC 16.1.1):

$ cmake --preset dev-mode
$ cmake --build build_dev_mode -t bitcoind
$ cmake --build build_dev_mode -t bitcoin-qt
[166/172] Building CXX object src/qt/CMakeFiles/bitcoinqt.dir/bitcoinqt_autogen/mocs_compilation.cpp.o
In file included from /home/hebasto/dev/bitcoin-gui/build_dev_mode/src/qt/bitcoinqt_autogen/EWIEGA46WW/moc_bitcoingui.cpp:9,
                 from /home/hebasto/dev/bitcoin-gui/build_dev_mode/src/qt/bitcoinqt_autogen/mocs_compilation.cpp:9:
/home/hebasto/dev/bitcoin-gui/src/qt/bitcoingui.h:67:7: warning: defining ‘BitcoinGUI’, which previously failed to be complete in a SFINAE context [-Wsfinae-incomplete=]
   67 | class BitcoinGUI : public QMainWindow
      |       ^~~~~~~~~~
In file included from /usr/include/qt6/QtCore/qobject.h:19,
                 from /usr/include/qt6/QtWidgets/qwidget.h:10,
                 from /usr/include/qt6/QtWidgets/qdialog.h:9,
                 from /usr/include/qt6/QtWidgets/QDialog:1,
                 from /home/hebasto/dev/bitcoin-gui/src/qt/addressbookpage.h:8,
                 from /home/hebasto/dev/bitcoin-gui/build_dev_mode/src/qt/bitcoinqt_autogen/EWIEGA46WW/moc_addressbookpage.cpp:9,
                 from /home/hebasto/dev/bitcoin-gui/build_dev_mode/src/qt/bitcoinqt_autogen/mocs_compilation.cpp:2:
/usr/include/qt6/QtCore/qmetatype.h:344:64: note: here.  Use ‘-Wsfinae-incomplete=2’ for a diagnostic at that point
  344 |         static auto check(U *) -> std::integral_constant<bool, sizeof(U) != 0>;
      |                                                                ^~~~~~~~~
In file included from /home/hebasto/dev/bitcoin-gui/src/qt/paymentserver.h:35,
                 from /home/hebasto/dev/bitcoin-gui/build_dev_mode/src/qt/bitcoinqt_autogen/EWIEGA46WW/moc_paymentserver.cpp:9,
                 from /home/hebasto/dev/bitcoin-gui/build_dev_mode/src/qt/bitcoinqt_autogen/mocs_compilation.cpp:27:
/home/hebasto/dev/bitcoin-gui/src/qt/sendcoinsrecipient.h:15:7: warning: defining ‘SendCoinsRecipient’, which previously failed to be complete in a SFINAE context [-Wsfinae-incomplete=]
   15 | class SendCoinsRecipient
      |       ^~~~~~~~~~~~~~~~~~
/usr/include/qt6/QtCore/qmetatype.h:344:64: note: here.  Use ‘-Wsfinae-incomplete=2’ for a diagnostic at that point
  344 |         static auto check(U *) -> std::integral_constant<bool, sizeof(U) != 0>;
      |                                                                ^~~~~~~~~
In file included from /home/hebasto/dev/bitcoin-gui/src/qt/psbtoperationsdialog.h:13,
                 from /home/hebasto/dev/bitcoin-gui/build_dev_mode/src/qt/bitcoinqt_autogen/EWIEGA46WW/moc_psbtoperationsdialog.cpp:9,
                 from /home/hebasto/dev/bitcoin-gui/build_dev_mode/src/qt/bitcoinqt_autogen/mocs_compilation.cpp:30:
/home/hebasto/dev/bitcoin-gui/src/qt/walletmodel.h:48:7: warning: defining ‘WalletModel’, which previously failed to be complete in a SFINAE context [-Wsfinae-incomplete=]
   48 | class WalletModel : public QObject
      |       ^~~~~~~~~~~
/usr/include/qt6/QtCore/qmetatype.h:344:64: note: here.  Use ‘-Wsfinae-incomplete=2’ for a diagnostic at that point
  344 |         static auto check(U *) -> std::integral_constant<bool, sizeof(U) != 0>;
      |                                                                ^~~~~~~~~
In file included from /home/hebasto/dev/bitcoin-gui/build_dev_mode/src/qt/bitcoinqt_autogen/EWIEGA46WW/moc_qvalidatedlineedit.cpp:9,
                 from /home/hebasto/dev/bitcoin-gui/build_dev_mode/src/qt/bitcoinqt_autogen/mocs_compilation.cpp:32:
/home/hebasto/dev/bitcoin-gui/src/qt/qvalidatedlineedit.h:13:7: warning: defining ‘QValidatedLineEdit’, which previously failed to be complete in a SFINAE context [-Wsfinae-incomplete=]
   13 | class QValidatedLineEdit : public QLineEdit
      |       ^~~~~~~~~~~~~~~~~~
/usr/include/qt6/QtCore/qmetatype.h:344:64: note: here.  Use ‘-Wsfinae-incomplete=2’ for a diagnostic at that point
  344 |         static auto check(U *) -> std::integral_constant<bool, sizeof(U) != 0>;
      |                                                                ^~~~~~~~~
In file included from /home/hebasto/dev/bitcoin-gui/build_dev_mode/src/qt/bitcoinqt_autogen/EWIEGA46WW/moc_rpcconsole.cpp:9,
                 from /home/hebasto/dev/bitcoin-gui/build_dev_mode/src/qt/bitcoinqt_autogen/mocs_compilation.cpp:37:
/home/hebasto/dev/bitcoin-gui/src/qt/rpcconsole.h:43:7: warning: defining ‘RPCConsole’, which previously failed to be complete in a SFINAE context [-Wsfinae-incomplete=]
   43 | class RPCConsole: public QWidget
      |       ^~~~~~~~~~
/usr/include/qt6/QtCore/qmetatype.h:344:64: note: here.  Use ‘-Wsfinae-incomplete=2’ for a diagnostic at that point
  344 |         static auto check(U *) -> std::integral_constant<bool, sizeof(U) != 0>;
      |                                                                ^~~~~~~~~
In file included from /home/hebasto/dev/bitcoin-gui/build_dev_mode/src/qt/bitcoinqt_autogen/EWIEGA46WW/moc_sendcoinsentry.cpp:9,
                 from /home/hebasto/dev/bitcoin-gui/build_dev_mode/src/qt/bitcoinqt_autogen/mocs_compilation.cpp:39:
/home/hebasto/dev/bitcoin-gui/src/qt/sendcoinsentry.h:26:7: warning: defining ‘SendCoinsEntry’, which previously failed to be complete in a SFINAE context [-Wsfinae-incomplete=]
   26 | class SendCoinsEntry : public QWidget
      |       ^~~~~~~~~~~~~~
/usr/include/qt6/QtCore/qmetatype.h:344:64: note: here.  Use ‘-Wsfinae-incomplete=2’ for a diagnostic at that point
  344 |         static auto check(U *) -> std::integral_constant<bool, sizeof(U) != 0>;
      |                                                                ^~~~~~~~~
[172/172] Linking CXX executable bin/bitcoin-qt

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🚧 At least one of the CI tasks failed. _{Task lint: https://github.com/bitcoin-core/gui/actions/runs/29695312658/job/88214942136} _{LLM reason (✨ experimental): CI failed because the lint-includes check detected C++ files including .cpp sources (triggering a lint “all_python_linters” failure).}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

</details>

Friendly ping @maflcko @l0rinc

Additionally:
1. Some of the `BitcoinGUI` class's private members are gated with
   `#ifdef ENABLE_WALLET` to prevent `-Wunused-private-field` warnings
   when building with `-DENABLE_WALLET=OFF`.

How to reproduce this?

Additionally:
1. Some of the `BitcoinGUI` class's private members are gated with
   `#ifdef ENABLE_WALLET` to prevent `-Wunused-private-field` warnings
   when building with `-DENABLE_WALLET=OFF`.

How to reproduce this?

See the CI log for the previous state of this branch: https://github.com/bitcoin-core/gui/actions/runs/29695312658/job/88214942182.

review ACK 51d36dfd076a98cd761ab312ab10792725ed7a29 🦅

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