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/qaudiosystem.h

@@ -97,6 +97,8 @@ public:
     virtual QAudioFormat format() const = 0;
     virtual void setVolume(qreal) {}
     virtual qreal volume() const { return 1.0; }
+    virtual QString category() const { return QString(); }
+    virtual void setCategory(const QString &) { }
 
 Q_SIGNALS:
     void errorChanged(QAudio::Error);