Configuration of output device for SDL

Let users configure the output device that's used by the SDL driver.
Code-wise the implementation is very similar to the input device
configuration.

Use a `QComboBox` instead of a `QLineEdit` for `m_device` and rename it
to `m_playbackDeviceComboBox`.

Rename `s_defaultInputDevice` to `s_systemDefaultDevice` because it is
used in the context of playback and input devices.

include/AudioSdl.h

@@ -39,7 +39,6 @@
 #include "AudioDevice.h"
 #include "AudioDeviceSetupWidget.h"
 
-class QLineEdit;
 class QComboBox;
 
 namespace lmms
@@ -67,10 +66,10 @@ public:
 		void saveSettings() override;
 
 	private:
-		QLineEdit * m_device;
+		QComboBox* m_playbackDeviceComboBox;
 		QComboBox* m_inputDeviceComboBox = nullptr;
 
-		static QString s_defaultInputDevice;
+		static QString s_systemDefaultDevice;
 	} ;