Bug 15910 - Move header search keep text JavaScript into staff-global.js

Bug 15910 - Move header search keep text JavaScript into staff-global.js

Bug 14189 added some global JavaScript to a new include file which is
included in doc-head-close.inc. Since it's included globally, there is
no reason not to move it to staff-global.js. This patch does so.

To test, apply the patch and clear your browser cache if necessary.

- Navigate to any page in the staff client which has multiple header
  search tabs.
- Type something in one of the tabs.
- Switch tabs. Verify that your text was copied to the newly-displayed
  form field.

Followed test plan, works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com