[ticket/11777] Search for template listeners in the php event listeners way

http://tracker.phpbb.com/browse/PHPBB3-11777

With the introduction of twig as a templating engine, search behavior for extension template event listeners changed and broke WIP extensions at the time. Instead of restoring the old situation, this pull request is intended to set things right by mimicking the way phpBB currently looks for php event listeners. Template listeners will have to be placed in 'styles/&lt;style&gt;/template/event/' and named '&lt;event name&gt;_listener.html'.