Even better thread and application termination:

- Now that timed-pop()/try_pop() prevent indefinite blocking it is best to flush() on thread stopping to unblock push()
- Reworked a bit the CubicSDR::OnExit() sequence, see comments.

src/process/FFTVisualDataThread.cpp

@@ -76,6 +76,9 @@ void FFTVisualDataThread::run() {
             wproc.run();
         }
     }
+
+    pipeIQDataIn->flush();
+    pipeFFTDataOut->flush();
     
 //    std::cout << "FFT visual data thread done." << std::endl;
 }