[ticket/12401] Pass more data to core.viewtopic_modify_post_row event

$topic_data array is used in viewtopic.php to populate $post_row template
block array data. Although $topic_data is not being passed
to core.viewtopic_modify_post_row event for modifying/adding
$post_row data in it. So, pass this array to the event.
Also, total_posts is added.

PHPBB3-12401
Merge pull request #2301 from rxu/ticket/12401

[ticket/12401] Add $topic_data array to core.viewtopic_modify_post_row event

* rxu/ticket/12401:
  [ticket/12401] Pass more data to core.viewtopic_modify_post_row event