More code cleanup

Added ACCESSOR_FUNC macro (creates a Set and Get function for the
specified class member) => Usage: ACCESSOR_FUNC(Name, std::string,
m_sPersonName);

Added World.GetSceneLayer(Layer)
Added World.GetTextureHolder (returns a pointer)
Updated Aircraft CTOR (removed texture holder argument)
Updated World.GetCommandQueue (returns a pointer)
Updated Player.HandleEvent
Fixed g_pWorld being NULL