Core: property API is polymorph (#472).

BREAKING CHANGE

The topic/association related property methods (get, set, has, remove) are polymorph. They are removed from the Core Service API and located now in the DeepaMehtaObject base class.

DeepaMehtaObject:
    - Object getProperty(String propUri);
    - void setProperty(String propUri, Object propValue, boolean addToIndex);
    - boolean hasProperty(String propUri);
    - void removeProperty(String propUri);

See #472.