FIR Polyphase filterbank channelizer prototype implementation

- Can now handle several 200khz FM streams with SDRPlay at 8Mhz+ on my
old 2010 Macbook Pro :)
- Demod bandwidth max now limited to 400khz, temporary until alternate
path for high-bandwidth is available

src/visual/TuningCanvas.cpp

@@ -190,8 +190,8 @@ void TuningCanvas::StepTuner(ActiveState state, int exponent, bool up) {
             bw += amount;
         }
 
-        if (bw > wxGetApp().getSampleRate()) {
-            bw = wxGetApp().getSampleRate();
+        if (bw > CHANNELIZER_RATE_MAX) {
+            bw = CHANNELIZER_RATE_MAX;
         }
 
         wxGetApp().getDemodMgr().setLastBandwidth(bw);