[ticket/13182] Add posting.php core event to allow modifying the message text

posting.php core event to allow modifying the message before parsing
would be needed for some extensions, like for adding topic title to the
topic URL etc.

<a href="https://tracker.phpbb.com/browse/PHPBB3-13182">PHPBB3-13182</a>.
This variable does not exist. Please also remove from the other events in this file.