Derived GameObjectInstantiator classes cannot fire base events

[LaneF]

When creating a derived class from GameObjectInstantiator and overriding AddPrimitive() it is not possible to fire the MeshAdded event due to rights scoping on event and there's not really a clean way to go about doing it otherwise.

I'm not sure what the implications of not firing are just yet, but this seemed like an oversight to me.