Defines a function to be called when this @{code Scene} is a drag and drop gesture source after its data has been dropped on a drop target. The transferMode of the event shows what just happened at the drop target. If transferMode has the value MOVE, then the source can clear out its data. Clearing the source's data gives the appropriate appearance to a user that the data has been moved by the drag and drop gesture. A transferMode that has the value NONE indicates that no data was transferred during the drag and drop gesture. Positional data for the DragEvent is invalid. Valid positional data for the DragEvent is presented in the onDragDropped event handler.