Form.setDefaultButton() supports only one button per form hierarchy

Currently Form.setDefaultButton() support only one button per whole form hierarchy, which is a problem when there are multiple forms on page of for example popup. It complicates even further when those default submit buttons have custom visibility/enabled state.

Current implementation puts hidden field in root form which stores name of submit button and then creates client JS click() via jquery.

This should be easy doable directly on form where the default button is located without the need to pass it to root form.

What do you think about this?

PR available here https://github.com/apache/wicket/pull/968