context: Add 'started' and 'prepare-shutdown' signals

The backend is changed to listen to 'prepare-shutdown' and forward, but
the 'started' signal will be used later.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2716>

src/backends/meta-backend-private.h

@@ -115,8 +115,6 @@ struct _MetaBackendClass
 
 void meta_backend_destroy (MetaBackend *backend);
 
-void meta_backend_prepare_shutdown (MetaBackend *backend);
-
 META_EXPORT_TEST
 ClutterBackend * meta_backend_get_clutter_backend (MetaBackend *backend);