clutter/timeline: Clear stage view listener when actor destroyed

We might have a stage view listener attached to the stage itself if the
actor didn't have a suitable frame clock when the actor was associated
with the timeline. We'd then listen to stage-views-changed signals on
the stage itself to be able to attach to a frame clock when one
appeared.

What went wrong is that if an actor that didn't have a frameclock was
associated with a timeline, but then destroyed, the timeline would
disassociate itself from the actor, but it'd still listen on the
stage-views-changed signal on the stage. This would be in itself
harmless, until the timeline itself is destroyed, as at this point, it
wouldn't clean up the stage-views-changed listener on the stage, as it's
assumed to only be valid when there is an actor attached.

Fix this issue by cleaning up the stage's stage-views-changed listener
when the actor is destroyed, as we wouldn't be able to make use of it by
then anyway.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3323
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1719>

clutter/clutter/clutter-timeline.c

@@ -311,6 +311,7 @@ on_actor_destroyed (ClutterActor    *actor,
 {
   ClutterTimelinePrivate *priv = timeline->priv;
 
+  g_clear_signal_handler (&priv->stage_stage_views_handler_id, priv->stage);
   priv->actor = NULL;
 }