Add the concept of shell capabilities, which allow the compositor to advertise support for the app menu and the global menubar, which are then propagated as GdkSettings. https://bugzilla.gnome.org/show_bug.cgi?id=707129
This protocol will be used by mutter-wayland and gtk to replace the _GTK X11 properties for DBus names/paths. https://bugzilla.gnome.org/show_bug.cgi?id=707129