Allow a function to be called when a target is "closed" / "finalized"

Upstream Qt issue https://bugreports.qt.io/browse/QTBUG-77377

Posting the description in here as well.

Such a feature would allow providing cleaner CMake API for users of Qt and CMake.

For example for pre-compiling QML files into some binary form, without this feature, you could have a hypothetical API as follows.

add_qml_module(Foo)
qml_target_sources(Foo a.qml)
if(APPLE)
  qml_target_sources(Foo b.qml)
endif()

more qml_target_source calls .....

qml_run_quick_compiler(Foo)

qml_run_quick_compiler needs to be called manually by a user after all qml_target_sources have been called.

With the proposed feature, we could have a cleaner API where the user does not have to call qml_run_quick_compiler explicitly, it would be called automatically by CMake.

add_qml_module(Foo)
qml_target_sources(Foo a.qml)
if(APPLE)
  qml_target_sources(Foo b.qml)
endif()

more qml_target_source calls .....

and inside add_qml_module you would have something like:

function(qml_run_quick_compiler target)
...
set_property(TARGET ${target} PROPERTY CALL_ON_TARGET_FINALIZE "qml_run_quick_compiler")
...
endfunction()

Thus the user can't accidentally forget to call qml_run_quick_compiler(), CMake would do it for them.

Of course we should find a nice design on how to specify that a function needs to be called, and how are parameters passed to it.