Bug 25261: (follow-up) Rename CircConfirmParts to CircConfirmItemParts

Bug 25261: (follow-up) Rename CircConfirmParts to CircConfirmItemParts

This clarifies the preference name to make it clear we are talking about
the 'parts' that make up an 'item'. 'Part' is a well known term in
british english libraries and I think perhaps 'Materials' may be
confused with other terms?

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>