the magic smoke connecting the dialog to the viewshell was in
SfxModalDialog, so not inheriting from that drops the connection

add a SfxDialogController to enable setting up the connection

Change-Id: I29c45cd43e05295309fc647b039db0c6565ca0d1
Reviewed-on: https://gerrit.libreoffice.org/61304
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>