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)
Although this is temporary, if we're landing with an l10n-id I think we should probably add the key in the .properties file. I don't think this should cause any translators unnecessary work as it's a bit early for 2.2 strings. But let's just add it to be consistent and we can remove it soon.