Bug 1074798 - Add in-between divider dropping hints. r=kgrandon

This simplifies the group rearrangement logic and brings it closer to spec.
Bug 1074798 - Indicate valid drops with group background colour. r=kgrandon
Bug 1074798 - Allow dropping an icon before the first group. r=kgrandon
Bug 1074798 - Allow dragging expanded groups. r=kgrandon

Collapses groups when you long-press to drag them, as per spec.
Bug 1074798 - Add a drop-shadow when dragging a group. r=kgrandon
Bug 1074798 - Handle injected contextmenu events in grid dragdrop. r=kgrandon