Bug 1082675 - Enable homescreen app-grouping permanently. r=kgrandon,crdlc

This reverts commit 071efd0eb9d962d4f96edc28d672e48910f48387.
Bug 1082675 - Fix divider->group related marionette test failures. r=crdlc,kgrandon

Some assumptions made by marionette tests no longer hold after enabling
app-grouping by default:

- Long-pressing a divider no longer brings up the context menu
- The divider element itself is not positioned (its children are)
Merge pull request #26006 from Cwiiis/bug1082675-app-grouping-by-default

Bug 1082675 - Enable homescreen app-grouping permanently. r=kgrandon,crdlc