Basic options for all sort criteria
Overrides SortPluginBase::buildOptionsForm
public function buildOptionsForm(&$form, &$form_state) {
parent::buildOptionsForm($form, $form_state);
$form['sort_within_level'] = array(
'#type' => 'checkbox',
'#title' => t('Sort within each hierarchy level'),
'#description' => t('Enable this to sort the items within each level of the hierarchy by weight and title. Warning: this may produce a slow query.'),
'#default_value' => $this->options['sort_within_level'],
);
}