Add experimental 'Live Preview' feature. On supported browsers, preview can be loaded via JavaScript so it doesn't disturb the surrounding page as badly. It's also lighter on the server, as it doesn't require skin rendering. (On short pages with Turck cache, this saves 50% of apache time!)

Disabled by default, as it's incomplete: interlanguage and category links
aren't displayed, and there are likely other problems.

Nonsupporting browsers or JS off fall back gracefully to form submission.
Tested: Safari 1.2.4, Firefox 1.0, MSIE/Win 6.0 (XPSP2)