Add support for Scene hierarchies

Scenes are currently presented in a flat list. It might be useful to allow the user to define a hierarchy for this list that would allow the scenes to be presented as:

```
- [group 1] V
--- [scene a]
--- [scene b]
--- [scene c]
- [group 2] >
- [group 3] V
--- [scene x]
--- [scene y]
--- [scene z]
```