Remove 'targetDB' irregularity and fix SaveButton widget.
Make "integer logarithmic" Range meaningful, use it for FFT resolution control.
Change FFT scale to /Hz rather than /bandwidth and align squelch hairline.

The scale of the FFT output of MonitorSink is now independent of the
source sample rate as well as the number of bins. Also fixed a forgotten
ref_scale value.
Avoid needing separate add_const block after logpwrfft.