Add tests for capabilities which have been updated to work
when only a model (and not whole objects) are cached; that
change was introduced to prevent misbehavior of the Remove
action in Edit mode, WTD-473.
Add a spec for an editable model cache; this will behave
similarly to the existing domain object cache, except
that it will cache only models. Use of this will permit
multiple instances of editable objects to be created with
unique contexts (that is, distinct parent objects accessible
via the context capability) which in turn will avoid the
ambiguous parentage which results in unexpected behavior of
the Remove action in Edit mode, WTD-473.
