LP#1611495 Don't use .xml for MARC21 files in Vandelay Export

LP#1611495 Don't use .xml for MARC21 files in Vandelay Export

The new 'Export Non-imported Records' feature in Vandelay (see bug
LP#1548147) was tweaked before merge to export MARC21 files by default
(rather than MARC-XML). However, the file save dialog still defaults to
(and filters by) ".xml" for the file extension. This is confusing and
otherwise problematic.

Let's go ahead and save the export using a ".mrc" extension instead.
While not an official extension (as far as I know), it is widely used,
easy to understand, and far less likely to cause issues with other
software.

Signed-off-by: Dan Wells <dbw2@calvin.edu>