Test of svg library.
Q: How should we interact with the svg canvas?
A: By sending JSON messages.
Q: What objects are clickabkle?
A: Only buttons. If we can make anthing clickabkle then it
will not be obvious.
Q: What objects are draggable?
A: Only drag squares. Again it should be obvious what objects are draggable.
Q: What are the drag methods?
A: on_drag_start/on_moved/on_dropped