Low priority: if PrintBox gets the equivalent of `hlist` that doesn't generate tables in HTML, move anchors to path lines

[lukstafi]

The overhead of tables on the size of generated HTML is non-trivial, assuming ~values_first_mode:true ~print_entry_ids:false is going to be the most frequent setup. It would be nicer to just output anchors with self-links on the path line -- going to the anchor would also unfold the subtree, and we wouldn't duplicate the presentation of entry ids.