Provide Qt5 <-> Q6 CMake compatible public API

We discussed this a few times at workshops, and concluded that we need to provide public CMake API that would allow mixing Qt5 and Qt6 packages, or allow easy migration/switching between Qt5 and Qt6 packages.

This involves:

• Provide versionless target names (Qt5::Core, Qt6::Core vs just Qt::Core)
• Allow configuring whether the versionless targets should be created
• Allow choosing the default version that versionless targets point to (Qt5 or Qt6) from the application build system
• Provide versionless qt_ prefix functions and macros
• Allow choosing which versions' implementation the functions will use
• Provide versionless targets for tools
  • (see at least tools created in qtbase/src/corelib/Qt5CoreConfigExtras.cmake.in and in qttools)
• Make enabled and disabled feature properties on targets to be compatible between Qt5 and Qt6 (same name)
  • This involves adding the Qt6 style properties to Qt5

Some things that are not yet clear:

• Do we need versionless targets for plugins?

A good test case for this will be QtCreator, to be able to flip a switch to build it either against Qt5 or Qt6.
There already exists a patch to emulates some parts that we would need to implement here https://codereview.qt-project.org/c/qt-creator/qt-creator/+/274901/8/cmake/FindQt5.cmake#57