Reset playbin state to NULL on end of stream signal.

According to GStreamer documentation "playbin should be set
back to READY or NULL state, then the "uri" property should be set
to the new location and then playbin be set to PLAYING state again."
We reset playbin to NULL state and then call setMedia() again
in case playback is restarted.

Change-Id: If7efbf8d88e0aad461c3d1d8b802c6621af221f7
Reviewed-by: Mithra Pattison <mithra.pattison@nokia.com>
Reviewed-by: Jonas Rabbe <jonas.rabbe@nokia.com>
Reviewed-by: Dmytro Poplavskiy <dmytro.poplavskiy@nokia.com>

src/plugins/gstreamer/mediaplayer/qgstreamerplayersession.h

@@ -131,6 +131,8 @@ public:
     void removeProbe(QGstreamerAudioProbeControl* probe);
     static gboolean padAudioBufferProbe(GstPad *pad, GstBuffer *buffer, gpointer user_data);
 
+    void endOfMediaReset();
+
 public slots:
     void loadFromUri(const QNetworkRequest &url);
     void loadFromStream(const QNetworkRequest &url, QIODevice *stream);