[ticket/13110] Add core event to the page_footer() function

Add core event to the page_footer() in includes/functions.php to allow
extensions handling overall page output before its displaying.

<a href="https://tracker.phpbb.com/browse/PHPBB3-13110">PHPBB3-13110</a>.
Yes, in case you would like to override default output behavior, f.e. filter the whole page before displaying the page etc., roughly
<div><pre> 
ob_start();
$this->template->display('body');
$content = ob_get_clean();
/* Do something with $content here */
$event['display_template'] = false;
</pre></div>