Get/Set the Object that event callbacks for this body will be sent to.
Care should be taken with any Object assigned as a callback target that isn't a Object as this assignment will not in itself keep the object alive and can be garbage collected.
To avoid this, you should have at least a single reference to the object in your code.
To remove the object assigned here, set the callback target to NULL.
This includes the following events:
- A BodyUpdateEvent with call IBodyUpdateCallback.