[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.

<a href="https://tracker.phpbb.com/browse/PHPBB3-12669">PHPBB3-12669</a>.
Please add forum_row, row, subforums_list and catless as arguments.

And please move it above `$last_catless = $catless;`