The intention is that this can be used as a grab bag of properties which
are not handled properly, yet we want the roundtrip of them to alien
formats.

See
http://lists.freedesktop.org/archives/libreoffice/2013-July/054428.html
for more details.

Change-Id: I3781b3b3bf1380d30683039f037d9a4292ba2f4a