Establish sane resource prefix for QML modules

We should add an option (RESOURCE_PREFIX auto) to qt_add_qml_module that sets the RESOURCE_PREFIX to /user.qt-project.org/imports. This prefix should then be added to the QML import path. This way we can easily identify well-behaved modules in their respective .qrc files. Furthermore we don't need to pollute the global .qrc namespace anymore. Modules the neither set a specific RESOURCE_PREFIX nor does "RESOURCE_PREFIX auto" should generate a warning at compile time.