audio: moved output to its own high priority thread

macOS audio playback failed due to audio being processed in the main thread.
Now all audio playback has been placed into its own high priority thread to
always keep up better with the operating system's demands.

app/audio/outputdeviceproxy.h

@@ -35,7 +35,7 @@ class AudioOutputDeviceProxy : public QIODevice
 {
   Q_OBJECT
 public:
-  AudioOutputDeviceProxy();
+  AudioOutputDeviceProxy() = default;
 
   virtual ~AudioOutputDeviceProxy() override;