CPCollectionView Issue 1755:

• Create the drop indicator in mouseEntered if needed. This is the case where the collection view is a drop destination but not a drag source.
• In _updateDragAndDropStateWithDraggingInfo... , handle the special case where the drop destination is 0 (includes item count == 0).
• Test app: add an empty collection view acting as a drop destination for the first collectionView.