[feature/event-dispatcher] Use real EventDispatcher through composer

* replace the copy-pasta EventDispatcher with the real one from Symfony2
* use composer for managing this dependency, use composer autoloading

PHPBB3-9550

phpBB/includes/event/data.php

@@ -15,7 +15,9 @@ if (!defined('IN_PHPBB'))
     exit;
 }
 
-class phpbb_event_data extends phpbb_event implements ArrayAccess
+use Symfony\Component\EventDispatcher\Event;
+
+class phpbb_event_data extends Event implements ArrayAccess
 {
     private $data;