Enable configuration of input device for SDL

Up to now the SDL audio driver attempted to use the default recording
device. This might not be what users want or expect, especially since the
actually used device is not visible anywhere. So if recording does not
work for the users they have no way to find out what's wrong.

Extend the settings screen of the SDL driver with a combo box that allows
to select the input device to be used. Store the selected device name in
a new attribute called "inputdevice" in the "audiosdl" section of the
configuration file.

Use the information from the configuration when attempting to inialize
the input device. Fall back to the default device if that does not work.

include/AudioSdl.h

@@ -40,6 +40,7 @@
 #include "AudioDeviceSetupWidget.h"
 
 class QLineEdit;
+class QComboBox;
 
 namespace lmms
 {
@@ -67,7 +68,7 @@ public:
 
 	private:
 		QLineEdit * m_device;
-
+		QComboBox* m_inputDeviceComboBox = nullptr;
 	} ;