backend: Store the type of the StageWindow implementation

We can use it later on to create it and provide a default create_stage()
implementation.

clutter/clutter-backend-private.h

@@ -53,6 +53,8 @@ struct _ClutterBackendClass
   /*< private >*/
   GObjectClass parent_class;
 
+  GType stage_window_type;
+
   /* vfuncs */
   gboolean              (* pre_parse)          (ClutterBackend  *backend,
                                                 GError         **error);

clutter/clutter-backend.c

@@ -512,6 +512,8 @@ clutter_backend_class_init (ClutterBackendClass *klass)
 
   g_type_class_add_private (gobject_class, sizeof (ClutterBackendPrivate));
 
+  klass->stage_window_type = G_TYPE_INVALID;
+
   /**
    * ClutterBackend::resolution-changed:
    * @backend: the #ClutterBackend that emitted the signal