Bug 32323: Correct focus state of some DataTables controls

Bug 32323: Correct focus state of some DataTables controls

This patch makes changes to the style of some DataTable controls in
order to make hover, active, and focus states more consistent.

To test, apply the patch and rebuild the staff interface css.

View a page that has a DataTable showing controls like "Columns,"
"Export," and "Configure." For example: Saved reports, Libraries,
Overdues.

 - Click the DataTables search form to put cursor focus there.
 - Use the tab key to move through the next buttons.
 - The style should now be more consistent with the appearance of other
   similar buttons in the staff client.
 - Test with and without text in the search filter.
 - Test pages which don't have the full set of controls, e.g. the
   table of of a patron's checkouts.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>