Provide a setting for system default input

Provide the setting "[System Default]" which instructs the SDL driver to
use the default device of the system as the input device. In the
configuration file this option is represented as an empty string. This
should play well with the current existing configuration of the users.

include/AudioSdl.h

@@ -69,6 +69,8 @@ public:
 	private:
 		QLineEdit * m_device;
 		QComboBox* m_inputDeviceComboBox = nullptr;
+
+		static QString s_defaultInputDevice;
 	} ;