#108 Frequency zoom on spectrum

[Craig Stuart Sapp]

It would be extremely useful to be able
to zoom and pan on the frequency (horizontal)
axis in the Spectrum display window.

I have no idea what the zoom dial on the Spectrum
layer is doing, but in any case it should be mapped
to the above behaviour in a manner identical to the
vertical zoom and pan wheel/slider on the spectrogram
display, since a spectrum display is a vertical
slice of the spectrogram display.