WIP: Homescreen grouping (v2)

Everything is in a group, dividers act as group separators.

Working (to a fashion):
- Rearranging groups
- Expanding/collapsing groups
- Creating new groups between groups
- Saving/restoring collapsed state
- Transitions

TODO:
- Editable group names
- General polish and optimisation (pending Gecko bugs)
I can use classList if you like, but I'm not sure what your rationale is? classList is a pretty recent thing and it seems if we're creating and controlling the element, it makes sense to use className and reduce the amount of code necessary.