[API CHANGE]: extends the css.configuration.XReadWriteAccess in an incompatible
way, but that is unpublished, has been commented as "still unpublished and
unstable," and was specifically introduced for internal use by the simplified
configuration access, so should not affect 3rd party code.

Change-Id: I99ce045f5bd8c598e689d46fb0d3626dfaa6d0a0