Qt: implement remainder of 'Options' menu

Source/Core/DolphinQt2/Config/SettingsWindow.h

@@ -13,10 +13,13 @@ class SettingsWindow final : public QDialog
   Q_OBJECT
 public:
   explicit SettingsWindow(QWidget* parent = nullptr);
+  void SelectAudioPane();
+
 signals:
   void EmulationStarted();
   void EmulationStopped();
 
 private:
   ListTabWidget* m_tabs;
+  int m_audio_pane_index = -1;
 };