Bug 28959: Add virtualshelves.public as a boolean

Bug 28959: Add virtualshelves.public as a boolean

This patchset moves the 'category' attribute for virtual shelves, that
takes values of 1 and 2 (private and public respectively) into a boolean
for public.

The DBRev is trivial, and the changes to the code are as well.

To test:
1. have some known public and private lists
2. Apply this patches
3. Run:
   $ updatedatabase
=> SUCCESS: Public lists have public=1, private have public=0
4. Run:
   $ kshell
  k$ prove t/db_dependent/Utils/Datatables_Virtualshelves.t \
           t/db_dependent/Virtualshelves.t
=> SUCCESS: Tests pass!
5. Try the feature in staff and OPAC
=> SUCCESS: All good
6. Sign off :-D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>