[ticket/13020] Add variables to core.viewforum_get_topic_data event

Event request: https://area51.phpbb.com/phpBB/viewtopic.php?f=111&t=45803
Also, forum template variables should be available for modifying to extensions.

<a href="https://tracker.phpbb.com/browse/PHPBB3-13020">PHPBB3-13020</a>.
Overwriting already assigned template var is kinda tricky task. Reassigning an array value is much easier.