Introduces 'css.url' callback event.

Allows extending the links 'txp:css' generates, enabling
changing stylesheets location if necessary. Styles could be
fetched from flat files for instance.

CSS link generation might get merged to the actual routing
and link generation code, in which case the event might
get renamed to 'url.css' or similar.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5233 2fea6a4d-a838-0410-917b-93a53c48e9d2