Port issue previewing code to the D6 API

From the deadwood.module conversion report on project_issue:

TODO: Node previews and adding form fields to the node form.

There is a subtle but important difference in the way node previews (and other such operations) are carried out when adding or editing a node. With the new Forms API, the node form is handled as a multi-step form. When the node form is previewed, all the form values are submitted, and the form is rebuilt with those form values put into $form['#node']. Thus, form elements that are added to the node form will lose any user input unless they set their '#default_value' elements using this embedded node object.

If you create a new issue and hit "Preview", you get:

Preview
Array

...