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.
Utilize model cache, permitting object models in edit mode to
be reused across multiple distinct instances with unique
contexts (unique contexts are necessary to avoid ambiguity in
the Remove action, WTD-473).
To avoid infinite digest cycles as a consequence of this,
refactor context/composition capability wrappers such that the
former is idempotent (since idempotence is no longer ensured
by the EditableDomainObjectCache) to avoid infinite digest
errors in Edit mode.
