Web: Improve presentation of description lists

* Announce entering and leaving list
* Announce item count (based on the number of description list terms)
* Append the role name at the end of each description list term in
  order to make it possible to distinguish terms from their descriptions
  in confusing lists, while still making it possible for users to cut
  off that speech in cases where the distinction is obvious.
* Similarly append the role name at the end of each description list
  value.

See issue #248