Prevent dragging from one list to another.

This could be done easily by using the `dataTransfer` object in `onDragOver` and
checking for a type... but for some reason that I can't determine, dataTransfer
is empty during dragOver, so in order to find out what's being dragged, I have
to use a props callback on the parent.
Test dataTransfer type keys for drag type rather than using parent elements.

Apparently during all drag events except for dragstart and drop, the
dataTransfer object exists in "Protected" mode, meaning: "For all other events.
The formats and kinds in the drag data store list of items representing dragged
data can be enumerated, but the data itself is unavailable and no new data can
be added."

Reference:
http://www.whatwg.org/specs/web-apps/current-work/multipage/dnd.html