[ticket/13005] Add event to display_forums() to modify category template data

While there're events to modify/add FORUM_POST/FORUM_LINK template data,
such an event is missing for FORUM_CAT forum type.

<a href="https://tracker.phpbb.com/browse/PHPBB3-13005">PHPBB3-13005</a>.
*whether the
 (same above)