app: show new version availability in About dialog.

I was wondering which shape should take the new version notification
(again some ugly pop-up?!) and realized using the About dialog was a
good idea.
This is preparatory work for this to happen.

(cherry picked from commit 826ece3b86)

app/dialogs/dialogs-constructors.c

@@ -195,7 +195,7 @@ dialogs_about_get (GimpDialogFactory *factory,
                    GimpUIManager     *ui_manager,
                    gint               view_size)
 {
-  return about_dialog_create (context);
+  return about_dialog_create (context->gimp->config);
 }
 
 GtkWidget *