Fix exception thrown in MARC editor searching for bib sources

Fix exception thrown in MARC editor searching for bib sources

As reported in https://bugs.launchpad.net/evergreen/+bug/695526,
the attempt to retrieve all of the config.bib_source values using
openils.PermaCrud was failing because the retrieveAll() method
relies on fieldmapper.IDL's setting of the
fieldmapper[hint].Identifier property for each class - but in the
1.6 releases, we're still using fmclasses in XUL instead of
fieldmapper.AutoIDL.

So, to fix the direct problem, we can simply set the property
ourselves on the desired class before invoking retrieveAll();
this avoids touching code paths that might have wider unintended
consequences.

git-svn-id: svn://svn.open-ils.org/ILS/branches/rel_1_6@19088 dcc99617-32d9-48b4-a31d-7c20da2025e4