apps-menu: Adjust to ClutterContainer removal

The `actor-added` signal got renamed to `child-added` when
moving it to ClutterActor.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/288>

extensions/apps-menu/extension.js

@@ -255,7 +255,7 @@ class DesktopTarget extends EventEmitter {
         this._desktopDestroyedId = 0;
 
         this._windowAddedId =
-            global.window_group.connect('actor-added',
+            global.window_group.connect('child-added',
                 this._onWindowAdded.bind(this));
 
         global.get_window_actors().forEach(a => {