Add BBCode system core and template events

This is a ticket for adding the events to the core and template files needed for the Advanced BBCode Box 3.1 extension.

Template events added to:
acp_bbcodes.html
acp_users_signature.html
prosilver and subsilver2 / posting_buttons.html

Core events added to:
acp_bbcodes.php
functions_display.php
message_parser.php

These events will allow the extension to update the ACP BBcode page to allow custom sorting of custom bbcodes, and to assign group-based permission/access to use of custom bbcodes.

They also allow the BBCode buttons to be replaced by a new BBCode button design/system.

And they allow for custom BBCodes that might require some extra processing after going through the BBCode engine