Fallback to default audio/MIDI backends if invalid one is set

Also shows the setup dialog in case of invalid audio backends

include/Mixer.h

@@ -308,6 +308,9 @@ public:
 	void requestChangeInModel();
 	void doneChangeInModel();
 
+	static bool isAudioDevNameValid(QString name);
+	static bool isMidiDevNameValid(QString name);
+
 
 signals:
 	void qualitySettingsChanged();