Re-factoring

Re-factoring

 - Re-factored / moved MapEngine::greatCircleDistance()
   to GeoCoordinate::distanceTo()

 - Implemented operator- for GeoCoordinate

 - Changed GPSPositionPrivate implementation to return always the
   accuracy got from the GeoPositionInfo, and not the
   GPS_ACCURACY_UNDEFINED value in case of network positioning.

 - Changed the GPSLocationItem inherit from QGraphicsItem instead of
   QGraphicsPixmapItem. Implemented GPSLocationItem::boundingRect()