Allow ckeditor to support multiple dialog afterclose callbacks

In core/modules/ckeditor/js/ckeditor.js ckeditor doesn't support saveCallbacks for multiple dialogs open, e.g. when one dialog opens another one and the latter is closed, saveCallback is set to null. Thus when the first one is closed then, the callback isn't called. The code causing the behaviour:

$(window).on('dialog:afterclose', function (e, dialog, $element) {
  if (Drupal.ckeditor.saveCallback) {
      Drupal.ckeditor.saveCallback = null;
  }
});