Dispose webview in DocumentViewerWindow

Elwig/Windows/DocumentViewerWindow.xaml.cs

@@ -18,6 +18,8 @@ namespace Elwig.Windows {
         }
 
         public void OnClosed(object sender, EventArgs evt) {
+            WebView.Stop();
+            WebView.Dispose();
             PdfFile?.Dispose();
             PdfFile = null;
         }