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/qgstreamerplayercontrol.h

@@ -144,6 +144,7 @@ private:
     int m_bufferProgress;
     bool m_seekToStartPending;
     qint64 m_pendingSeekPosition;
+    bool m_setMediaPending;
     QMediaContent m_currentResource;
     QIODevice *m_stream;
     QSocketNotifier *m_fifoNotifier;