When the user drops a draggable on the DragTarget widget,
the onAccept callback is invoked. This is when you get
to decide whether or not to accept the data that was dropped.
In this case, the item is always accepted and processed. 
You might choose to inspect the incoming item to make a
different decision.

Notice that the type of item dropped on DragTarget
must match the type of the item dragged from LongPressDraggable.
If the types are not compatible, then 
the onAccept method isn’t invoked.

With a DragTarget widget configured to accept your
desired data, you can now transmit data from one part
of your UI to another by dragging and dropping.

In the next step,
you update the customer’s cart with the dropped menu item.

Add a menu item to a cart

Each customer is represented by a Customer object,
which maintains a cart of items and a price total.