Add a sliding spectrogram mode

[unfa]

I like the simplicity and elegance of the look of this program.
It'd be great if it could work as a scrolling spectrogram, to show the history of the sound spectrum, not only instantaneous FFT analysis.

[naught101]

Yaaas. That would be super useful. Kind of difficult in multi-channel mode, but maybe the waterfall plot could just be the mixed spectrum of all channels?