Instead of running macdeployqt once for each extra helper binary and
plugin (plus the main run for the application itself), collect all the
binaries that should be processed and run macdeloyqt only once after
all the binaries have been installed.
This also moves the main app macdeployqt call from a POST_BUILD step
to an install(CODE) step, making increment rebuilds of the app faster.
To ensure that macdeployqt is called after all the binaries are
installed, a new post_install subdirectory is needed to
circumvent CMake's limitation regarding the default order of
installation.
CMake first runs the current directory install() calls and then it's
child subdirectory ones. Because we want macdeployqt to be the last
install() call, it needs to be done inside a subdirectory that is
added last.
Note due to a bug in macdeployqt, the deployed app inside the .dmg
file will fail to run on arm macs, due to broken code signature.
See https://bugreports.qt.io/browse/QTBUG-101696 for details.
For the final release, the release-tool should take care of proper
resigning.
