Database file is now handled by each page, instead of a central place

Opening a database while another one is open triggers a warning message

ModernKeePass/App.xaml.cs

@@ -199,7 +199,7 @@ namespace ModernKeePass
             base.OnFileActivated(args);
             var rootFrame = new Frame();
             var file = args.Files[0] as StorageFile;
-            rootFrame.Navigate(typeof(OpenDatabasePage), file);
+            rootFrame.Navigate(typeof(MainPage), file);
             Window.Current.Content = rootFrame;
             Window.Current.Activate();
         }