Bug 22694: Add Koha::Patron::Category->override_hidden_items

Bug 22694: Add Koha::Patron::Category->override_hidden_items

This patch introduces a method to determine for a given category if
patrons belonging to it override OpacHiddenItems policies.

To test:
- Apply the patches
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Patron/Category.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: Bin Wen <bin.wen@inlibro.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>