Add core event "core.modify_submit_post_data"

[ticket/12190] Add core event - core.modify_submit_post_data
to posting.php. This allows extensions to modify the whole post
data before the post submitting, or even provide its own post
submit procedure.

<a href="http://tracker.phpbb.com/browse/PHPBB3-12190">PHPBB3-12190</a>.