webstaff: tweak how element IDs in MARC editor are generated

webstaff: tweak how element IDs in MARC editor are generated

For new records (or records that for whatever reason lack
a 901 field), use '0' as a dummy value for the record
ID when constructing element IDs (e.g., r0f5, r0f10sf0value,
etc.).  This fixes a problem where setCaret() couldn't
successfully change focus after adding a field or subfield
to a new record.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Kathy Lussier <klussier@masslnc.org>