[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.

<a href="https://tracker.phpbb.com/browse/PHPBB3-12828">PHPBB3-12828</a>.
there is a `trigger_error($message)` in each of the `if ($submit)`-blocks