Add viewforum.php core event to allow modifying topics data before display the page

Identifier: core.viewforum_modify_topics_data
*Location:*viewforum.php
Explanation: the event allows extensions to use and/or modify topics list and/or topics data before dumping out the viewforum page.
Use case: For extensions which operate with viewforum page, f.e. if the topics list and data is needed to retrieve, add, modify additional topics info before the output loop.