gwt - model objects agnostic of presenters

Removed dependency from ToDoItem to ToDoPresenter.
It's better if model objects do not invoke methods on presenters or views. One reason is that in GWT, model classes usually lives under a `shared.*` package, since many model objects are transferred back and forth between client and server through RPC calls (RemoteService's). By making model objects depend on client side code, does not allow them to be exposed on these interfaces.