actor: Call unparent() in destroy

If the actor is an internal child of another actor then we should call
unparent() when destroying it, like clutter_actor_reparent() does;
otherwise we'll leak the actor, since the parent holds a reference to
it.

http://bugzilla.openedhand.com/show_bug.cgi?id=2009

clutter/clutter-actor.c

@@ -3067,7 +3067,7 @@ clutter_actor_dispose (GObject *object)
           clutter_container_remove_actor (CLUTTER_CONTAINER (parent), self);
         }
       else
-        priv->parent_actor = NULL;
+        clutter_actor_unparent (self);
     }
 
   /* parent should be gone */