* Add warning in document tiddler toolbar if tiddler already exists
* Live preview document in new window
* Fix slicer.js bug that was preventing the list field of headings from
being filled in correctly
* Rationalise some class names
Now includes a special document view column on the left. Headings can
be expanded/collapsed, and tiddler titles can be inspected and renamed
via the toolbar. Clicking on an entry opens the associated tiddler. The
default tiddler view template includes a special section for tiddlers
that are part of a document
