Add ability to add items to menus in a sorted way

If you want to register menu entries in a modularized application like described in https://symfony.com/bundles/KnpMenuBundle/current/events.html the menu items will be added in the order the event listeners are executed. So basically random from an application standpoint. When adding menu items by event listeners I need to be able to control the order of the menu items when adding them, because at menu retrieval the information encapsulated in a module is no longer known.

Start with the menu event-listener documentation at https://symfony.com/bundles/KnpMenuBundle/current/events.html and trace how listeners add items before menu retrieval. Define how callers can provide ordering information while preserving modular event registration, then verify that items are returned in the requested order.

Written by the indexing model from the issue text.