AGC for the SSB demod
Added an AGC on the channel filtered signal so that the audio volume does not vary following the signal power from station to station or due to QSB. Also added a power threshold squelch so that background noise does not rise at normal signal level when there is no signal. This is really necessary in a multi-channel (multi-plugin) configuration.
More details in the SSB demodulator readme: https://github.com/f4exb/sdrangel/blob/master/plugins/channelrx/demodssb/readme.md
Other changes
- ChannelAnalyzerNG: improved LSB handling in the GUI