Hacky Fix: Changing frames by setting Scene.frame_current directly didn't update rigs in 3D viewport

Operators did not suffer from this problem as they were still just using notifiers
directly. The "proper" fix is to use the new message bus system. But, we've
got enough problems dealing with COW already as it is now... moving on.

source/blender/makesrna/intern/rna_scene.c

@@ -884,6 +884,7 @@ static void rna_Scene_frame_update(Main *bmain, Scene *UNUSED(current_scene), Po
 {
 	Scene *scene = (Scene *)ptr->id.data;
 	BKE_sound_seek_scene(bmain, scene);
+	WM_main_add_notifier(NC_SCENE | ND_FRAME, scene);
 }
 
 static PointerRNA rna_Scene_active_keying_set_get(PointerRNA *ptr)