Load child topics in response filter (#699).

BREAKING CHANGE

Core REST API: if the `fetch_composite` query parameter is not present in the request `false` is assumed. That is, by deafult the response entity contains no child topics. If child topics are wanted `fetch_composite=true` must be stated explicitely.
(In DM 4.3 and before `fetch_composite=true` was the default for **single item** responses and `fetch_composite=false` was the default for **item collections** responses. These differentiation is no abolished in favor of better remembrance.)

All REST APIs: if the request has a `fetch_composite=true` query parameter the child topics of the response entity are loaded automatically. For standard cases the plugin developer must not care about child topic loading. Child topic loading is performed generically by the Core's `JerseyResponseFilter`.

Automatic child topic loading works for responses of these types:
    - `Topic`
    - `Association`
    - `Iterable<Topic>`

If the response is not of one of these types the plugin developer must care about manual child topic loading. She does so by using `DeepaMehtaObject`'s `loadChildTopics()` methods.

See #699.