Provide a way to customize the "prefer" declaration in auto-generated qmldir files

Auto-generated qmldir files currently contain a preferred path that's hard-coded to point to the embedded qrc location.

There are some use-cases where users have resorted to manually removing the "prefer" line afterwards (see QTBUG-120435)

Those use-cases could be helped by allowing users to provide a custom "prefer" declaration (with the understanding that pointing to the non-default location means losing the Qt Quick Compiler optimizations)

Notes

• Unlike QTBUG-138268, this idea should be compatible with QTP0004 in general
• But perhaps of implementing QTBUG-138268 as a separate keyword, the "prefer" directive could be dropped by setting the custom path to an empty path (then the warnings in QTBUG-138268 would apply)