Retain classes of buttons added to modal dialog with data-buttons

modal.js allows buttons to be added to the modal dialog by setting a `data-buttons` attribute on the modal link: https://github.com/processwire/processwire/blob/eddd6cb8ad5974196ae10b84b032a03a7c94ed97/wire/modules/Jquery/JqueryUI/modal.js#L22-L24

This request is that the added buttons retain the classes of the original buttons, so that the added buttons can be targeted with CSS and JS.