Draggable: dropOnEmpty option or Sortable: repurpose same

I found a (brief!) discussion on whether dropOnEmpty should exist on Sortable. The answer was no.

It is useful; but it's the wrong way around. Currently in order to prevent a connected Draggable from being dropped on an empty sortable you have to disable the sortable whenever it becomes empty and re-enable it if it gains content (on the theory that drag+drop is not the only way of creating DOM content).

I would propose that dropOnEmpty should apply to the sortable itself, as to whether it will accept a droppable if it is empty; rather than whether its own items can be dropped onto empty others.

Alternatively, the options to Draggable should have a dropOnEmpty that does the same thing as Sortable's; i.e. prevents this draggable being dropped on an empty sortable.

The former, while preferable in my mind, has the disadvantage of not being backward-compatible.