Bug 26057: Add 'cancel' method to Koha::Item::Transfer

Bug 26057: Add 'cancel' method to Koha::Item::Transfer

This patch adds the 'cancel' method to Koha::Item::Transfer which sets
the transfer as cancelled by updating the datecancelled field. We also
update Koha::Item->get_transfer here to accomodate for the new
resolution available for a transfer.

Test plan:
1/ Run the included unit tests additions (t/db_dependent/Koha/Items.t,
t/db_dependent/Koha/Item/Transfer.t

Signed-off-by: Kathleen Milne <kathleen.milne@cne-siar.gov.uk>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>