Bug 1022414 - [Vertical] Dragdrop improvements

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.

https://bugzilla.mozilla.org/show_bug.cgi?id=1022414
Yeah, I'll update it to be the same as layout.cols, nice catch.