Visualize time and frequency domains

[sowbug]

It should be easy to stick a spectrum analyzer into something, and to see the amplitude shape over time.

[sowbug]

Depends on #7

[sowbug]

I've written widgets for these two. They need work:

• Not integrated into the app.
• We're computing the FFT on every render, which is not necessarily what we want. Instead, we need a companion struct that the app can ask to recompute the FFT less often than every update.
• Not pretty.
• Needs various parameters.

[sowbug]

Tracking #151 and #152