the gen vclplug, which is then unused for the gtk/gtk3 case
so only possible to use in gen/kde.

Make the default impl the no-op case and override just in
gen

Change-Id: Id0dd3fe4ae4e5b94bfc56219defab56905ec710a
Reviewed-on: https://gerrit.libreoffice.org/28675Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>