DLG_MAILMERGE conversion to .ui

Change-Id: I9f62470666dfdc2e1a3e636ccc1893871b8e8bd4
Reviewed-on: https://gerrit.libreoffice.org/10753
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>