[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.
Also removed 'delete' var from core.posting_modify_template_vars and
core.modify_posting_parameters as it does not exist.

PHPBB3-13182
Merge pull request #3043 from rxu/ticket/13182

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