DCOM-33: Allow to register callbacks in the EventManager

[doctrinebot]

Jira issue originally created by user @beberlei:

[doctrinebot]

Comment created by @beberlei:

Consider to add a second queue for callbacks. Its not possible to simulate this by doing something like:

public function addCallbackListener($event, Closure $callback)
{
    $eventListener = new stdClass();
    $eventListener->$event = $callback;
    $this->addEventListener($event, $eventListener);
}

sad :-)