Another fix for the neverending hung application bugs:

- Clear up DemodulatorInstance garbage collection, only do it in a single thread (AppFrame::OnIdle)
- Add a specific mutex lock for the list of deleted demodulators,
- Add a specific lock to control DemodulatorInstance thread lifetimes, protecting IsTerminated() of concurrent join and delete

src/process/VisualProcessor.h

@@ -170,7 +170,7 @@ protected:
             if (inp) {
                 OutputDataTypePtr outp = buffers.getBuffer();
 
-                //'deep copy of the contents 
+                //'deep copy' of the contents 
                 (*outp) = (*inp);
   
                 VisualProcessor<OutputDataType, OutputDataType>::distribute(outp);