Allow nested reorder components (stop propagation on item down)

[cyberpluto]

This allows to work with nested Reorder components, otherwise all nested elements drag together :)

[JakeSidSmith]

@cyberpluto I wouldn't want to add this to the library in this way as there would be no way to not prevent the stopPropagation. However, allowing a user to manually call stopPropagation would be fine. Or perhaps a prop that stops propagation.

Adding a callback for onItemDown would allow a user to manually call this. Would that be a suitable solution for you?