Entities should inherit inverse attributes

Depends on issue #198

edit: or not?

``` C++
        test_inverse_attr::e_window->AddSupertype(test_inverse_attr::e_object);
        test_inverse_attr::e_object->AddSubtype(test_inverse_attr::e_window);
```

Maybe just iterate over the supertype list, which is returned by `EntityDescriptor::GetSupertypes()`
