I want to implement a custom Track Sub-Group. Inheriting from UnityEngine.Timeline.GroupTrack doesn't allow to add children to it. I just can't figure it out what I have to write to make that work. How can I implement a custom GroupTrack that allows me to add child-tracks to it? It doesn't need any mixing whatsoever, it just need to group a bunch of tracks to run some code at runtime over it. PS: I'm using Unity 2021.3 and Timeline 1.6.4.