MainWindow: Add feedback for users when App.CheckForUpdates finds no updates

Elwig/Windows/MainWindow.xaml.cs

@@ -41,7 +41,7 @@ namespace Elwig.Windows {
         }
 
         private async void Menu_Help_Update_Click(object sender, RoutedEventArgs evt) {
-            await App.CheckForUpdates();
+            await App.CheckForUpdates(true);
         }
 
         private async void Menu_Help_Smtp_Click(object sender, RoutedEventArgs evt) {