delaying FormOverlay.loadFormContentAndRender() till the widget is needed

It is difficult to defer loading the form for the FormOverlay until a user clicks on a link, since it doesn't give you access to the success handler, and handling the io:complete event is problematic since you have to ignore any other io:complete events for different pages or widgets.

For example, this code snippet would make the io call first, but before the asynchronous io call completed, the widget would be shown, so it would appear briefly without any content. Also, it is confusing that you have to call render() on the widget, since the method loadFormContentAndRender() only calls renderUI(), which is insufficient.

  link.on('click', function() {
    form_overlay.loadFormContentAndRender();
    form_overlay.render();
    form_overlay.show();
  }