Core: fix composite value model again (#339).

BREAKING CHANGE

- CompositeValueModel public API:

Again, semantics of add() method has slightly changed: if the topic is already added nothing is performed **unless** its ID is -1.

This allows adding a couple of new child topics at once e.g. in a webclient form by pressing the "Add" button serveral times. (was broken by recent commit)

Your plugins are probably *not* required to be updated.

See ticket 339.