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)
Could be? it is faster to set to false than delete an attribute
```js
this.active = active;
active ? this.element.classList.add('active') : this.element.classList.remove('active');
```