Fires at the end of the widget control form.

Use this hook to add extra fields to the widget form. The hook
is only fired if the value passed to the ‘widget_form_callback’
hook is not false.

Note: If the widget has no form, the text echoed from the default
form method can be hidden using CSS.

do_action_ref_array( 'in_widget_form', array( &$this, &$return, $instance ) );