configure_audio: Move combo box index setting to their own functions

Keeps the individual behaviors in their own functions, and cleanly
separate. We can also do a little better by converting the relevant IDs
within the core to a QString only once, instead of converting every
string into a std::string.

src/citra_qt/configuration/configure_audio.h

@@ -26,6 +26,8 @@ public slots:
 
 private:
     void setConfiguration();
+    void setOutputSinkFromSinkID();
+    void setAudioDeviceFromDeviceID();
     void setVolumeIndicatorText(int percentage);
 
     std::unique_ptr<Ui::ConfigureAudio> ui;