Save debug mail merge documents

This saves the first three mail merge documents, the source and
the corresponding working documents.

For convenience all saved document URIs printed to the console.

Change-Id: Ice86bf59fc519cfab170ff0956559dc9373d5a26
Reviewed-on: https://gerrit.libreoffice.org/9860
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>