Called, as needed, to build list item widgets.
List items are only built when they're scrolled into view.
The IndexedWidgetBuilder index parameter indicates the item's position in the list. The value of the index parameter will be between zero and one less than itemCount. All items in the list must have a unique Key, and should have some kind of listener to start the drag (usually a ReorderableDragStartListener or ReorderableDelayedDragStartListener).
final IndexedWidgetBuilder itemBuilder;