Gracefully handle deletion of source node while dragging

[gaearon]

I noticed that if I remove drag source node while drag operation is in progress, it never finishes. The reason for this is dragend not firing on removed elements.

We should perhaps add top-level window drop handler around here to check if drag source node was removed from DOM. In this case, we should somehow invoke pending handleDragEnd because the component might still be mounted (even though one of inner nodes was removed).

We need to think if we should invoke handleDragEnd if the component itself is unmounted by the time it happens. Probably not, but this would break beginDrag/endDrag symmetry. Another option is to invoke endDrag in componentWillUnmount with didDrop=false if there is a pending drag operation by the time component is unmounted.