Added QAudioDecoderControl::sourceChanged signal.

The signal is mentioned in QAudioDecoder but is missing from
QAudioDecoderControl.

Change-Id: I0cd13c53541585098edd02093858501ffb5af1fb
Reviewed-by: Ling Hu <ling.hu@nokia.com>
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>
Reviewed-by: Dmytro Poplavskiy <dmytro.poplavskiy@nokia.com>

src/multimedia/controls/qaudiodecodercontrol_p.cpp

@@ -173,6 +173,14 @@ QAudioDecoderControl::QAudioDecoderControl(QObject *parent):
     Signals that a new buffer is ready for reading.
 */
 
+/*!
+    \fn QAudioDecoderControl::sourceChanged()
+
+    Signals that the current source of the decoder has changed.
+
+    \sa sourceFilename(), sourceDevice()
+*/
+
 /*!
     \fn QAudioDecoderControl::formatChanged(const QAudioFormat &format)
 

src/multimedia/controls/qaudiodecodercontrol_p.h

@@ -83,6 +83,7 @@ public:
 Q_SIGNALS:
     void stateChanged(QAudioDecoder::State newState);
     void formatChanged(const QAudioFormat &format);
+    void sourceChanged();
 
     void error(int error, const QString &errorString);