Introduces additional steps to "admin_side" callback event.

Adds callback events:

* "admin_side > main_content"
* "admin_side > main_content_end"

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5250 2fea6a4d-a838-0410-917b-93a53c48e9d2
$txp_user isn't used in end_page().

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5251 2fea6a4d-a838-0410-917b-93a53c48e9d2