Bug 16139: Renaming 'Unseen since' column to 'Last seen'

Bug 16139: Renaming 'Unseen since' column to 'Last seen'

To test:
1) Go to Tools -> Inventory/Stocktaking
2) Submit empty form to bring back all records (or specify, up to you)
3) Notice column name 'Unseen since'
4) Apply patch and go back to Inventory/Stocktaking, resubmit form
5) Confirm column name is now 'Last seen' and makes more sense

Sponsored-by: Catalyst IT
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