MM: refactor cancel handling for dbui dialogs

This simplifies the handling of CreateMonitor and PrintMonitor in
the combined mail merge function.

Change-Id: Iacf933ec1a6017949b29b3d5f62d5f55c4691053
Reviewed-on: https://gerrit.libreoffice.org/10984
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>