App wizards: Simplify generated CMakeLists.txt

The current CMakeLists.txt files that are generated by the wizards try to work both for Qt 5 and Qt 6 versions. Unfortunately this leads to a lot of if()..else() blocks, and crute logic.

It would be arguably cleaner to let the user select the target version first, and then generate a CMakeLists.txt file that is taylored towards Qt 5 or Qt 6. Or provide a qt5compat.cmake shim that allows to write a modern CMakeLists.txt file that also works with older Qt 5 versions.