+ add XSD attributes "alias", "connection", "event", "method", "priority" for type "tag"
  see:
    http://symfony.com/doc/current/cookbook/doctrine/event_listeners_subscribers.html
    http://symfony.com/doc/current/components/dependency_injection/tags.html
    http://symfony.com/doc/current/cookbook/service_container/event_listener.html
+ add XSD attributes "alias", "connection", "event", "method", "priority" for type "tag"
  see:
    http://symfony.com/doc/current/cookbook/doctrine/event_listeners_subscribers.html
    http://symfony.com/doc/current/components/dependency_injection/tags.html
    http://symfony.com/doc/current/cookbook/service_container/event_listener.html