Bug 28821: (follow-up) Improve operation of button plus/less

Bug 28821: (follow-up) Improve operation of button plus/less

Follow-up for bug 28784.

Do not remove the op select on the first row.
Allow plus/less on all lines. Allow less until we have one line left.
Make sure that first op is disabled and hidden with plus/less.

Test plan:
Search on multiple indexes on advanced search.
Verify that plus/minus works as expected.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>