AudioPulseAudio: fixed improper shutdown + free allocated resources

There have been some problems with the threading logic in the
AudioPulseAudio backend resulting in an endless loop when quitting LMMS.

Furthermore allocated PulseAudio resources were not freed properly.
Thanks to Armin Kazmi <armin.kazmi@tu-dortmund.de> for pointing out
this issue.
(cherry picked from commit 168805745e)

include/AudioPulseAudio.h

@@ -79,6 +79,7 @@ private:
 	virtual void applyQualitySettings();
 	virtual void run();
 
+	volatile bool m_quit;
 
 	bool m_convertEndian;