Bug 22455: Add Koha::Biblio::hidden_in_opac method

Bug 22455: Add Koha::Biblio::hidden_in_opac method

This patch adds a hidden_in_opac method that does the same calculation
done in places like opac-tags.pl. The condition that is checked is that
all items belonging to the biblio are hidden. This is the current
behaviour in the code.

To test:
- Apply this patches
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Biblio.t
=> SUCCESS: Tests pass!

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>