Tween track

I see where you use this in the demo, but this method should be probably removed. The construction process should be:
```java
DragAndDrop tracksDragNDrop = new DragAndDrop();
TrackLayout t1 = new TrackLayout(tracksDragNDrop);
TrackLayout t2 = new TrackLayout(tracksDragNDrop);
```
These makes clear that the drag and drop is shared across tracks.