/// <summary>
 /// Navigiert zu einer Seite.
 /// </summary>
 /// <param name="resourceKey">Schlüssel aus dem ResourceDictionary der Seite, zu der navigiert werden soll</param>
 public void NavigateTo(string resourceKey)
 {
     if (!resourceKey.Equals(String.Empty))
     {
         var page = App.Current.FindResource(resourceKey) as Page;
         PageChanged?.Invoke(this, new NavigationEventArgs()
         {
             ZielPage = page,
         });
         (App.Current.FindResource("MainWindowVM") as MainWindowVM).CurrentPage = page;
         PageChangedFinished?.Invoke(this, new EventArgs());
     }
 }
 //DB fertig
 public void StartUpDone()
 {
     PageChangedFinished?.Invoke(this, new EventArgs());
 }