Having the Ability to Validate Drag

[john-grear]

Similar to #4657, validating drag would be a great way to make the drag-drop tree component better. Right now you can tell it when it should / should not drop, but there's no way (afaik) that you can make it display like it can't be dropped on something when dragging over it. Seems like it would be nearly the same way that the validate-drop is handled, calling event.accept(true) or event.accept(false) which triggers event.preventDefault() in the background.