Core: build attached composite values (#339).

Not yet usable.

Core API:
    de.deepamehta.core.ChildTopics is new interface (still empty).
ChildTopicsModel:
    Object get(String key) is new method -- Returns the TopicModel or List<TopicModel> for the given child type URI.

BREAKING CHANGES

ChildTopicsModel:
    Object get(String key) -- Convenience method for accessing the *simple* value of a single-valued child.
        is renamed to
    Object getObject(String key)

IMPORTANT: if your plugin makes use compValue.get("..") to access a simple value the call MUST be renamed to getObject() (see above). A get(..) method with the same signature still exists but has changed semantics (as described above). So, the need for method renaming can not be detected by the compiler.

See ticket 339.