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
I wonder if we should just do something like this in grid_item.js?
```
this.element.classList.add('type-' + this.detail.type);
```