so we can specify which window modal dialogs are modal to

Change-Id: I7cd7348b5cd0f55d698553f313f745d5e0d6389d
Reviewed-on: https://gerrit.libreoffice.org/67765
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>