[ticket/12177] Add ucp_friend_list_before/after

Add events before and after list of friends in UCP template

Events:
ucp_friend_list_before - before the friend list output.
ucp_friend_list_after - coresponding after friend list ouput.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-12177

P.S.: hope the formating is OK!