Everything has a NodeRenderer now

I think this covers all the interesting cases, but there are a couple of remaining questions. See the comments.
Btw on the topic of better names, I was thinking of `ShowTree` or `RenderTree` for `NodeRenderer`, and `RenderedTree` for `RenderedNode`. Does that seem more or less confusing? The `render` method could also be changed to `showTree` if that would help.