Signature for building children of a DragTarget.
candidateData argument contains the list of drag data that is hovering
over this DragTarget and that has passed DragTarget.onWillAccept. The
rejectedData argument contains the list of drag data that is hovering over
this DragTarget and that will not be accepted by the DragTarget.
Used by DragTarget.builder.
typedef DragTargetBuilder<T> = Widget Function(BuildContext context, List<T?> candidateData, List<dynamic> rejectedData);