Fix threshold in spectrogram -- it wasn't working in the last release. There is a new protocol for this. Formerly the threshold parameter had a range from -50dB to 0 with the default at -50, and -50 treated internally as "no threshold". However, there was a hardcoded, hidden internal threshold for spectrogram colour mapping at -80dB with anything below this being rounded to zero. Now the threshold parameter has range -81 to -1 with the default at -80, -81 is treated internally as "no threshold", and there is no hidden internal threshold. So the default behaviour is the same as before, an effective -80dB threshold, but it is now possible to change this in both directions. Sessions reloaded from prior versions may look slightly different because, if the session says there should be no threshold, there will now actually be no threshold instead of having the hidden internal one. Still need to do something in the UI to make it apparent that the -81dB setting removes the threshold entirely. This is at least no worse than the previous, also obscured, magic -50dB setting.