Bring back PortAudio support on Windows (#4770)

Population of the PortAudio setup widget has been delayed until it is shown, as it was causing crashes on some systems.

include/AudioPortAudio.h

@@ -34,6 +34,7 @@ class AudioPortAudioSetupUtil : public QObject
 {
 	Q_OBJECT
 public slots:
+	void updateBackends();
 	void updateDevices();
 	void updateChannels();
 
@@ -87,6 +88,7 @@ public:
 		virtual ~setupWidget();
 
 		virtual void saveSettings();
+		virtual void show();
 
 	private:
 		ComboBox * m_backend;