Add functional Spectrum view to demodulator visuals

- Might need to do some renaming from Scope->AVDisplay or something for
ScopeCanvas to avoid confusion.

src/audio/AudioThread.h

@@ -14,6 +14,7 @@
 class AudioThreadInput: public ReferenceCounter {
 public:
     long long frequency;
+    int inputRate;
     int sampleRate;
     int channels;
     float peak;