places-menu: Stop using compatibility actor property

PanelMenu.Button sets up a `this.actor = this` property for compatibility,
but let's reflect the actual new object hierarchy.

https://gitlab.gnome.org/GNOME/gnome-shell-extensions/issues/113

extensions/places-menu/extension.js

@@ -86,7 +86,7 @@ class PlacesMenu extends PanelMenu.Button {
                                    y_align: Clutter.ActorAlign.CENTER });
         hbox.add_child(label);
         hbox.add_child(PopupMenu.arrowIcon(St.Side.BOTTOM));
-        this.actor.add_actor(hbox);
+        this.add_actor(hbox);
 
         this.placesManager = new PlaceDisplay.PlacesManager();