Adjust behavior of add_qt_gui_executable / qt_add_executable as per API review

Resulted from https://codereview.qt-project.org/c/qt/qtbase/+/317353/1/src/corelib/Qt6CoreMacros.cmake#449

Things to implement:

1) Remove WIN32 MACOSX_BUNDLE properties.
Users should opt-in manually via set_target_properties.
This most likely implies that we'll need to regenerate all examples to set those where appropriate.

2) remove Gui linkage
I don't expect many changes needed in the examples to adapt to this.

3) Remove calling of qt6_generate_win32_rc_file
Not sure about the impact of this yet.