ListView drop indicator fixes (#3135)
* add root drop indicator hidden button and fix role
* fix root drop target render condition
* add hidden button for dropping on item
* don't render insertion indicator if not dragging
* add a second folder to drop-on-item example
* update 'root only' to allow dropping on own list
* fix insertion indicator z-index
* fix test using updated story
* fix tests
Co-authored-by: Daniel Lu <dl1644@gmail.com>
