Instead of showing a sequence of all documents in the preview column,
we just show a single document, with a dropdown to choose which one is
shown.
The change makes it easier to deal with large numbers of documents.
The metadata for each field is in a tiddler tagged
`$:/tags/DocumentMetaData` with the field `caption` being the displayed
caption for the field and `field` being the name of the field.
`field-type` is the type of the field, and can be “string” or “list”.
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
The exclude filter `+[tag[intro]]` will produce a document that only
includes the paragraphs with the tag “intro”. These are derived from
the paragraphs in the original document with the CSS class “intro”.
