Fixture display hammering

[taw10]

At present, the fixture display works by hammering the REPL with requests for status updates. It works, and the resource usage is OK for the shows so far. However, it doesn't scale at all.

The fixture display should not use polling, but rather some kind of event-based system. This would have the additional benefit of making the display more responsive, e.g. to selection changes.

[taw10]

Should be improved with cf8b7c8. Now, the combined state is requested once instead of tracing the state stack every time. There's no longer a separate re-calculation just for the fixture display. However, there is still room for further big improvements.