Map the PlainText UNO property to:
<loext:content-control loext:plain-text="...">
on export, and do the opposite on import.
Change-Id: Icec0c35b2b9fca53104e6526c98083db52df5d42
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/137340
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins