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)
maybe we could save it in a var
```js
var length = this.gridView.items.length;
if (tIndex !== 0 && tIndex !== length - 1) {

for (; tIndex < length - 1 && this.gridView.items[tIndex - 1].detail.type !== 'divider';
```