Bug 19935: Add Koha::Patron->pending_checkouts

Bug 19935: Add Koha::Patron->pending_checkouts

To move this subroutine out of the C4 namespace we face the same
problematic as bug 17553 (with GetOverduesForPatron).
We need to provide an equivalent method and so return all the related
value for a given checkout.
We can acchieve the easily using Koha::Object->unblessed_all_relateds,
but we need to keep in mind that it is a temporary move.
Indeed we will want to use our API to only access/retrive values we really need.
The whole trick could be removed when the current syntax for notices
will be deprecated and removed.

Note: this method returns the same number of elements than ->checkouts
They indeed returns the same things, but it sounds better to me to have a
different method to highlight (from the callers) where does it come
from (C4::Members::GetPendingIssues).

Signed-off-by: Benjamin Rokseth <benjamin.rokseth@deichman.no>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>