Improved spectrum analyzer mode

- Faster sweeps by changing 2.LO only when necessary and using 400kHz I2C frequency
- Added FPGA settings for selectable ADC samplerate
- Additional measurement with different ADC samplerate when signal ID is on to remove ADC images

Software/PC_Application/Traces/tracebodeplot.cpp

@@ -146,6 +146,8 @@ TraceBodePlot::TraceBodePlot(TraceModel &model, QWidget *parent)
     // enable autoscaling and set for full span (no information about actual span available yet)
     setXAxis(0, 6000000000);
     setXAxis(true, 0, 6000000000, 600000000);
+    // get notified when the span changes
+    connect(&model, &TraceModel::SpanChanged, this, qOverload<double, double>(&TraceBodePlot::setXAxis));
 }
 
 TraceBodePlot::~TraceBodePlot()