Move string_tocallback to Textpattern_Type_String.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5543 2fea6a4d-a838-0410-917b-93a53c48e9d2
Commit preferences panel too.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5544 2fea6a4d-a838-0410-917b-93a53c48e9d2