Bug 11518: Add new method to K::S::R::Item that will always return the correct itemtype

Bug 11518: Add new method to K::S::R::Item that will always return the correct itemtype

There are many disparate areas of Koha that deal with item level
itemtypes vs record level itemtypes. We can take advantage of
DBIx::Class to make smarter objects that automatically return the
correct value depending on the system preference.

Test Plan:
1) Apply this patch
2) Run t/db_dependent/Items.t

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>