Rewrite the dataflow graph code #19

There's a not lot to like about this code:

• Adding binding events to every label individually seems clumsy.
• FixedUnbindMixin works but maybe we could make it unnecessary?
• FlippyCanvas is a pretty weird thing to be split out, and should probably just be part of the behaviour of a better GraphWrapper.
• I'm not convinced that the UI decisions here are all for the best.

This issue exists because I'm not going to think about any of these things until v0.1.x is released.