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.
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
