workaround for dragend not firing on elements removed from dom

Workaround for issue #37.

More information:
https://bugzilla.mozilla.org/show_bug.cgi?id=460801

Possible alternative described here:
https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Drag_operations#dragend
but would require a convention of never removing source element from dom inside onDrop.