Add pointer events

[parnic]

This exists to work around a conflict in the Flutter engine where
multiple Listeners at different levels in the widget hierarchy cannot
work together.

See flutter/flutter#76525 et al

[parnic]

I was trying to hook up the ability to drag an item outside of the DragAndDropLists widget and still be able to scroll the outer scrollview around (in my case, the DragAndDropLists widget is embedded in a page that's doing a lot of other things as well, so the ability to scroll the entire page while dragging was critical). It would be nice to remove this after the Flutter team fixes the fundamental issue with multiple listeners not cooperating.