1. Add one field, parent_menu, which should have the same name as the folder name (case insensitive). For menus has the same parent_menu are supposed to have the same menu_order.
 2. Change the way to generate navigator. Now it traverses the menu_order. If it is list of menu where the first element has parent_menu, it will generate a pop-up parent menu with a list of nav. Otherwise it will generate nav as before.