Act as if the object being edited is the root object when
in Edit mode, except in the Library pane (which has its own
means of getting the real root object.) WTD-922.
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.
