With #13809 (packaging split off), we lost the .desktop and .protocol files.
This PR restores them, and puts them in their proper places as part of make install. (Systems without XDG can be configured --without-xdg to avoid this, and the macOS/Windows gitian descriptors are updated to use it.)
Note that:
bitcoin128.svgneeds to be named such because (for some reason)bitcoin128has become the de facto standard XDG icon name for the generic Bitcoin icon.org.bitcoin.bitcoin-qt.desktopis required to matchQAPP_ORG_DOMAIN, which we have set to"bitcoin.org"for some reason. (Changing that is out of scope for this PR)