[ticket/12669] Add core event to the function display_forums()

Add core event to the function display_forums()
in includes/functions_display.php to allow assigning additional template data
for every forum, f.e. additional forumrow subloops.

PHPBB3-12669
Merge pull request #2554 from rxu/ticket/12669

[ticket/12669] Add core event to the function display_forums()

* rxu/ticket/12669:
  [ticket/12669] Add core event to the function display_forums()