Refactoring of the render part of the webform component.

* Actual rendering is done in hook_form_webform_client_form_alter to
  instead of during form process (which is done a lot more often).
* Show a placeholder in form_builder instead of trying to figure
  everything out in _webform_render_[component]().
* All changes should be API and DB compatible.
* Insert @TODO items for API-breaking changes.