DLG_MM_ASSIGNFIELDS conversion to .ui

Change-Id: Ia625de9edd46c185c2712c0c5a7154ba1abb2894
Reviewed-on: https://gerrit.libreoffice.org/9726
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>