Upgrade for dart-dnd v0.1.1

Thanks @marcojakob for this really awesome library.

I see you've been doing some work lately on dart-dnd, so this is an attempt to bring the dart-swiper library up-to-date for dnd's latest changes.
The `cancelled` flag was to indicate in the `DraggableEvent` if the user normally ended the drag operation or if he cancelled. Cancelling could be done by clicking the `esc` key or when the window looses focus.

Well, I thought we wouldn't need that flag and removed it, but maybe it would still be useful. I think it's better if the swiper goes back to the previous position if the user clicks `esc` - don't you?