citra_qt: Add a volume slider

src/core/settings.h

@@ -139,6 +139,7 @@ struct Values {
     std::string sink_id;
     bool enable_audio_stretching;
     std::string audio_device_id;
+    float volume;
 
     // Camera
     std::array<std::string, Service::CAM::NumCameras> camera_name;