Removed "addActionConverter" from our Application

* We now handle it ourselves in the ActionsParamsConvertersPacker, with a "converter-" Silex callback name prefix
* As we often sort Arrays by priority, added a method for that in ArrayUtils :-)
Added a "Actions firewalls" system

* this is just a maybe-easier-to-understand-for-non-developers name for Silex "before actions" middlewares
Added a global "actions.run" app function
Added a "authentification required" Actions firewall
Sign-in/up/out now works correctly in Ajax mode