Many reasons:
* to allow subtrees to be grafted more easily
* to keep the tags for an entry clean by removing structural tags and
leaving the semantic tags
* to avoid the duplication of expressing the same relationship through
both the tags and list fields
1. Switch from using the text field of lists for storing the associated
filter to using the field `toc-list-filter` (to make it harder to
accidentally parse the text of an ordinary tiddler as a filter)
2. Fix several bugs
* 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
