[ticket/12828] Add ucp_prefs.php common core event

Add common core event at the bottom of the includes/ucp/ucp_prefs.php
to allow additional actions (like template variable assignments etc.)
before the page load.

PHPBB3-12828
Merge pull request #2709 from rxu/ticket/12828

[ticket/12828] Add ucp_prefs.php common core event

* rxu/ticket/12828:
  [ticket/12828] Add ucp_prefs.php common core event