Sign Message UI allows selection of keyless addresses #17969

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<!-- Describe the issue -->

What Happens

When you click File|Sign Message, the UI asks you to choose an address with which to sign. You can enter one by hand, paste from clipboard, or "Choose a previously used address." The third option provides a list of addresses that includes addresses for which there is no private key (P2SH addresses). This is intimately related to #10542 but the fix here is straightforward.

<!--- What behavior did you expect? -->

Expected Behavior

I expect addresses for which no private key is known to be excluded from the list presented. I realize that the same list is used to select an address to which to receive a payment, but that code could be adjusted to accept a filter, or the private-key-less addresses could be filtered out during display.

<!--- What was the actual behavior (provide screenshots if the issue is GUI-related)? -->

To reproduce

Ensure that you have at least one P2SH address in your Bitcoin core wallet. Select File | Sign Message Click the address book. Note that there are P2SH addresses offered for selection.

This is in Bitcoin Core 0.18.

Hey should we check this exists: PKHash* pkhash = boost::get<PKHash>(&addressinlist); And then would we have to call the wallet().getPrivKey to check that as well or would the above be enough. Actually would it not be more efficient to have a filtering bool value for the addresses, or is there another location we can fetch them from

I've moved this over to the GUI repo: https://github.com/bitcoin-core/gui/issues/62.