This is similar to the
officecfg::Office::Common::Save::Document::GenerateThumbnail config
setting, but here we allow configuring this at a per-save basis, not
persistently.

Change-Id: Ieb5bd57f1d8fc9e211011f2647276d985cf53131
Reviewed-on: https://gerrit.libreoffice.org/47812Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>