[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.

PHPBB3-13005
Merge pull request #2901 from rxu/ticket/13005

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

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