Use this field to declare a submit field in your form.
has_field submit => ( type => Submit, value => Save );
It will be used by HTML::FormHandler::Render::Simple to construct
a form with $form->render.
Uses the submit widget.
If you have multiple submit buttons, currently the only way to test
which one has been clicked is with $field->input. The value
attribute is used for the HTML input field value.