Bug 26635: Refined data structure and methods

Bug 26635: Refined data structure and methods

This patch makes the returned data structure be simpler:

_str => {
    attribute_1 => {
        category => 'some_category_name',
        str      => 'description',
        type     => 'av'
    },
    ...
}

The description is sensible to context, so if public => 1 is passed,
then lib_opac is passed, and lib is returned otherwise. Whenever we add
language to the combo, we will add it to the implementation.

Tests are adjusted accordingly, also to reflect the public => 1 use
case.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Object.t \
           t/db_dependent/Koha/REST/Plugin/Objects.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>