popupMenu: Make MenuManager to take an actor as parameter

Since we assume now that most of the classes are actors per se we can just pass
the actor itself as parameter

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/487

js/gdm/loginDialog.js

@@ -325,7 +325,7 @@ var SessionMenuButton = class {
                  this._button.remove_style_pseudo_class('active');
         });
 
-        this._manager = new PopupMenu.PopupMenuManager({ actor: this._button },
+        this._manager = new PopupMenu.PopupMenuManager(this._button,
                                                        { actionMode: Shell.ActionMode.NONE });
         this._manager.addMenu(this._menu);