Add threshold to drag detection, to reduce over sensitivity

[bentoogood]

Users are finding that it is too easy to accidentally drag an item from a list.

[johnhaddon]

This does need doing, but I think we can address the actual problem the users are having as well. They're slipping and accidentally dragging a single selected field in a list of paths onto the list of paths itself - which is removing all paths except the one dropped. We should just ignore drags which are originating on the same widget as they're being dropped on (making sure you can still drop onto the "-" button).