Bug 16726: Clear text in syspref searchbox after submitting

Bug 16726: Clear text in syspref searchbox after submitting

To test:
1) Go to Admin -> search for a system preference
2) Notice your search stays in the search box (this is inconsistent with
    search behaviour across Koha)
3) Apply patch and refresh page
4) Make another search
5) Confirm search still works as expected and search terms have been
cleared from search box
6) Confirm search terms show at the top of the results

Sponsored-by: Catalyst IT
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>