fix the private shelf deletion in opac

fix the private shelf deletion in opac

this patch add an "hidden" field in the template, that permit in C4::VirtualShelves::Pages to get the right(privates) shelves with
GetShelves in$
of always public shelves.
So when a private shelf is wanted to be deleted, the script watch only in public shelves if the shelf exists, and doesn't work.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>