Properly handle the case where QtWidgets is not available.

It does use a config test for now, just to make it clearer.  The Mac
backend currently requires widgets, but most of the others should work
without it (just not supporting QVideoWidget/WindowControl).

The QVideoDeviceControl interface was modified to remove the QIcon
usage - it was never implemented.

Unfortunately even the QML examples need widgets for the wrapper
application, and will need to be ported to QtGui based wrapper.

Change-Id: I8a55ad5cf09ab51749510bf16f49de0bd3f0bcdb
Reviewed-by: Ling Hu <ling.hu@nokia.com>

src/plugins/gstreamer/mediaplayer/qgstreamerplayerservice.h

@@ -80,8 +80,10 @@ private:
 
     QMediaControl *m_videoOutput;
     QMediaControl *m_videoRenderer;
+#if defined(HAVE_XVIDEO) && defined(HAVE_WIDGETS)
     QMediaControl *m_videoWindow;
     QMediaControl *m_videoWidget;
+#endif
 };
 
 QT_END_NAMESPACE