Investigate and add in better lifecycle support for game entities

[bijington]

Consider letting GameScene and GameObject know when child objects are added/removed. Also allow for objects to know when they have been added/removed.

And finally GameScene should know when it is loaded/unloaded.

Something like:

public class GameScene
{
    public void OnGameObjectAdded(IGameObject gameObject);

    public void OnGameObjectRemoved(IGameObject gameObject);

    public void OnLoaded();

    public void OnUnloaded();
}

public class GameObject
{
    public void OnGameObjectAdded(IGameObject gameObject);

    public void OnGameObjectRemoved(IGameObject gameObject);

    public void OnAdded(); // TODO: Parent?

    public void OnRemoved();
}