Bug 1067435 - Homescreen grouping. r=crdlc,kgrandon

Everything is in a group, dividers act as group separators.

Features:
- Rearranging groups
- Expanding/collapsing groups
- Creating new groups between groups
- Saving/restoring collapsed state
Bug 1067435 - Add unit tests for GaiaGrid group feature. r=kgrandon,crdlc
Bug 1067435 - Add a setting for homescreen app-grouping. r=kgrandon,crdlc
Merge pull request #24057 from Cwiiis/proto-grouping-2

Bug 1067435 - Homescreen grouping. r=crdlc,kgrandon