Save and restore audio gain settings.
Reconnect sniffer when switching demod chains

This prevents the application from aborting when the AFSK decoder window is closed after changing modes while the decoder window is open.
Merge pull request #63 from nhasan/master

Save and restore audio gain setting.
Make receiver::rx==NULL a valid and the inital state of a receiver object
Tell build tools to always link to gnuradio-audio
Merge branch 'master' of https://github.com/csete/gqrx