Bug 991460 - Allow adding apps to collections in verticalhome. r=kgrandon

- Make consts in grid_dragdrop upper case.
- Add a 'hovered' class to icons being hovered over in GaiaGrid
- Add a 'collection' class to collections in a GaiaGrid
- Add a larger rearrange delay when hovering over a collection
- Add a pulsing animation to collections when they're hovered over
- Add hook to drop icons into collections
- Add IAC to add apps to collections
Merge pull request #20798 from KevinGrandon/Cwiiis-bug991460-squashed

Bug 991460 - Allow adding apps to collections in verticalhome.