Bug 991460 - Allow adding apps to collections in verticalhome.

- 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

https://bugzilla.mozilla.org/show_bug.cgi?id=991460

Travis run: https://travis-ci.org/KevinGrandon/gaia/builds/28088564