Add experimental 'Live Preview' feature. On supported browsers, preview

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)