Write output from a background thread

[sourcefrog]

Currently, every call to View::update updates the model and then may, or may not, write output to stdout (or wherever) depending on rate limiting.

It might be good to instead have a thread dedicated to pushing updates:

1. The application thread that provided the update can return to its own real work faster, without potentially blocking writing to a slow terminal. (However, since updates are rate-limited, these writes should happen rarely and might not have much of an effect? Or they might?)
2. If a progress bar update happens within the cooldown from the last update, it has missed its chance to be painted: the progress bar will not repaint until there's another model update. Similarly, after a message is written the progress bar will not repaint until there's a new model update. This isn't a problem for applications that issue many updates, but for those that update sporadically it can be strange: the bar doesn't show the thing that's actually causing a long pause. Asynchronous repainting would let the bar paint every n milliseconds with whatever is the most current state.

I'm not sure if messages should also be painted by an asynchronous thread, presumably with some queue in between.