Collapsible/expandible, can change root, and full user interface.
See the gist in action at http://bl.ocks.org/wmleler/raw/a734fb2bb3319a2cb386/.
There is also a help file.
The source is at https://bl.ocks.org/wmleler/a734fb2bb3319a2cb386/.
I didn’t need drag and drop, but it could be added with not too much trouble.