[ticket/9550] Add the core.viewtopic_post_rowset_data event to viewtopic.php

http://tracker.phpbb.com/browse/PHPBB3-9550

Also see http://area51.phpbb.com/phpBB/viewtopic.php?f=111&t=44511

To allow extra fields added to the query retrieving post data to be used for setting template variables, they need to be added to the post rowset. This pull request adds the core.viewtopic_post_rowset_data event that enables extension developers to modify the post rowset when the database row is still available.