Bug 18433: Allow to select results to export in item search

Bug 18433: Allow to select results to export in item search

This adds a column of checkboxes in the results table to be able to select the
items to be exported

Test plan:
1. Go to item search and click 'Search'
2. Without checking any checkbox, verify that the export still works (it should
   export all results)
3. Tick some checkboxes and re-export, verify that only selected items are
   exported

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>