Bug 23889: Improve style of menu header in advanced cataloging editor

Bug 23889: Improve style of menu header in advanced cataloging editor

This patch updates the style of dropdown menu headers to make it apply
more globally and changes the advanced cataloging editor template so
that the correct class is applied to the menu header.

To test, apply the patch and rebuild the staff client CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

Test the interfaces where these kinds of menu headers can be found:

 - Catalog search results: The "Sort" menu (Popularity, Author, etc.
   headings).

 - Saved report results: Run a report which returns a column of
   biblionumbers or itemnumbers. Check the "Batch operations" menu
   (Bibliographic records or item records headings).

 - Advanced cataloging editor: The settings menu (Change framework
   header).

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>