Bug 22300: Staff search results: Opt groups in 'sort' pull down are not well formatted

Bug 22300: Staff search results: Opt groups in 'sort' pull down are not well formatted

This patch adds some CSS to style the staff client search results sort
menu headings. Formerly these were <optgroup> tags with a default
browser style.

To test, apply the patch and regenerate the staff client CSS.

 - Perform a catalog search in the staff client.
 - On the search results page, test the "Sort" menu. Confirm that the
   menu headers more clearly indicate the sections.
 - Test the "Add to list" button menu too, where the same style will be
   in use.

Signed-off-by: Pierre-Marc Thibault <pierre-marc.thibault@inLibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>