public PresentationViewModel(
     string path,
     int initialSlide,
     PresentationProgressObserver progressObserver)
 {
     _progressObserver = progressObserver;
     File         = path;
     CurrentSlide = initialSlide;
 }
   public PresentationViewModel(
 string path, 
 int initialSlide, 
 PresentationProgressObserver progressObserver)
   {
       _progressObserver = progressObserver;
         File = path;
         CurrentSlide = initialSlide;
   }