[ticket/13294] Add function parse() core event for additional message checks

Add core event to the function parse() in includes/message_parser.php to allow
additional handling message before parsing (i.e. perform custom message checks,
cleanup etc.).

<a href="https://tracker.phpbb.com/browse/PHPBB3-13294">PHPBB3-13294</a>.
please add $this->warn_msg and the `return (!$update_this_message) ? $return_message : $this->warn_msg;` check (the event should allow the listener to deny the post, maybe with an additional var or by comparing warn_msg with its previous value,I don't know)