[mms] Use csstidy library to parse CSS files when creating static CSS (Bug #12043).

Should be more efficient, since we don't have to search for url
parameters three different times.

Also, should be more robust (fixes support for multiple urls in a single
CSS style).

Side effect: $conf['cachecssparams']['compress'] is no longer needed -
since we are already necessarily parsing the CSS, there's no reason not
to compress when saving.  Will remove the option in Horde 5.1 for
clarity.