Core: add more accessors to attached comp (#339).

CompositeValue (interface) has 3 new accessor methods:
    - Object get(String childTypeUri)
    - Iterable<String> childTypeUris()
    - int size()

Note: under some circumstances the underlying CompositeValueModel differs from the attached CompositeValue.
The 3 new accessors (as well as the existing has() method) operate on the attached values.

See ticket 339.