Bug 1022414 - [Vertical] Dragdrop improvements r=crdlc

Many improvements to drag drop in vertical homescreen including:
* Ignores dividers during distance calculation and relys only on icons or placeholders to determine proper drop target.
* Render a row of placeholders after the last divider. This ensures we have a valid drop target even if we ignore dividers.
* The last row of placeholders have a smaller size are are only for divider creation.
* Buffers from/to indexes to have slightly better performance and prevent the stuck icon situation.