Expose the audio category information for streams.

QAudioOutput and QSoundEffect now have a category property so that
system volume mixing or processing can be applied.

Initially just pulseaudio supports this but Windows Vista etc should also
work.

Change-Id: I6855b08367e5a055ac7dfcffd644c98bfd7c5a4e
Reviewed-by: Ling Hu <ling.hu@nokia.com>

src/multimedia/audio/qaudiooutput.h

@@ -100,6 +100,9 @@ public:
     void setVolume(qreal);
     qreal volume() const;
 
+    QString category() const;
+    void setCategory(const QString &category);
+
 Q_SIGNALS:
     void stateChanged(QAudio::State);
     void notify();