Additional flag to fix DragAndDrop in ScrollPane

I've noticed that it's not really possible to use `DragAndDrop` inside a `ScrollPane` currently. Since the scroll pane is higher in the hierarchy, it receives the touch down events before it could be handled by the `DragListener`. That's why usually a drag will start and after dragging the payload a little bit more, the scroll pane behaviour will kick in, scroll the pane and cancel the dragging.

I've added an additional flag which will enable users to cancel the touch focus for all other listeners, but the currently active `DragListener`, in case a successful drag event has begun (payload != null). I think this is better default behaviour, but I didn't want to change the current behaviour, so it's disabled by default.