Support Multiple SortableTrees on one Page

[ncphillips]

Current implementation does not elegantly handle multiple SortableTrees to be on the same page.

Dragging an item from one list to another causes serious problems, by deleting items in the other tree.

We need to be able to name the SortableTree. That name should be added to the drag type, to prevent items from one tree being dragged to other trees.

[ncphillips]

I'm working on fix for this.

[fritz-c]

The fix is now available in v0.1.6.
Thanks for bringing this to my attention!

[ncphillips]

Thanks for merging it @fritz-c!

And again, I had to move things around a bit and I bad because I wasn't really sure how you would want that done.