While MetaStage, MetaWindowGroup and MetaDBusDisplayConfigSkeleton don't
appear explicitly in the public API, their gtypes are still exposed via
meta_get_stage_for_screen(), meta_get_*window_group_for_screen() and
MetaMonitorManager's parent type. Newer versions of gjs will warn about
undefined properties if it encounters a gtype without introspection
information, so expose those types to shut up the warnings.
https://bugzilla.gnome.org/show_bug.cgi?id=781471
