popupMenu: Close all child menus when the main menu is closed

Otherwise, if something managed to call close(), it wouldn't
close any child menus.

https://bugzilla.gnome.org/show_bug.cgi?id=693572

js/ui/popupMenu.js

@@ -1267,6 +1267,10 @@ const PopupMenu = new Lang.Class({
         if (this._activeMenuItem)
             this._activeMenuItem.setActive(false);
 
+        this._childMenus.forEach(function(childMenu) {
+            childMenu.close();
+        });
+
         if (this._boxPointer.actor.visible)
             this._boxPointer.hide(animate);