Bug 14510: Allow column visibility in additem.pl to be customized

Bug 14510: Allow column visibility in additem.pl to be customized

This patch adds the table of items on additem.pl to the columns
customizer.

Test plan:
  1. Open item editor on a record, and verify that all columns are visible.
  2. Apply patch.
  3. Reload editor, and verify that column visibility hasn't changed.
  4. Open "Hide/show columns," and verify that you can add and remove
     columns.
  5. Change the visibility and togglability of some columns in
     columns_settings.pl, and verify that these correctly apply to
     additem.pl.

NOTE: The columns that are configurable are selected from the non-hidden
columns that have mappings to MARC subfields in the default MARC21
framework (and can thus be displayed in the item editor).

Signed-off-by: Jenny Schmidt <jschmidt@switchinc.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>