New layout for topiclist lists

New layout for .topiclist lists

Adjustments for .html files include:
- Wrapping content of dt in div.list-inner
- Adding extra tab for contents of div.list-inner
- Adding class name to ul.topiclist to indicate how many columns are there

RFC: http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=44139
Ticket: http://tracker.phpbb.com/browse/PHPBB3-11489