Add ability to add items to menus in a sorted way

[B4rb4ross4]

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.

[B4rb4ross4]

My suggestion: #381