New CMake API for customizing qmldir files

This started with the desire to control the "prefer" declaration (see QTBUG-138268 and QTBUG-138269). fabiankosmale commented in the former:

I see the use case, but together with QTBUG-138269 we have a strong hint that NO_QMLDIR_PREFERRED_PATH is not actually what's needed here (too unflexible/we'll end up with even more options in qt_add_qml_module). I strongly suspect that a combination on NO_GENERATE_QMLDIR + a new (public) function which takes care of qmldir generation (and which can have additional options) would be the better approach.

So, this suggestion is an alternative solution to QTBUG-138269 (although I haven't closed that yet, in case the QML team changes their mind)