Bug 20217: Make header's catalog search menu into a split button

Bug 20217: Make header's catalog search menu into a split button

This patch modifies the markup for the staff client header's catalog
search menu so that it mimics the behavior of a Bootstrap split button
(As seen on the cataloging home page's 'New from Z39.50' button).

To test, apply the patch, run 'yarn build' to regenerate the staff
client's CSS, and clear your browser cache.

- View any page in the staff client.
- Hover your mouse over the "Search" link in the header. It should
  trigger a hover affect on the adjacent menu link.
- Clicking the "Search" link should take you directly to the advanced
  search page.
- Clicking the adjacent menu link should display a menu containing links
  to advanced search and item search.

Signed-off-by: Claire Gravely <claire.gravely@bsz-bw.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>