Web: Handle description lists with extra stuff in the accessibility tree

Description lists can have their term+value items inside divs, e.g.
for styling. When that occurs, the browsers seem to be pruning the
extra divs from the accessibility tree (i.e. making them just a simple
description list from our perspective). However, if the author does
something to force inclusion of those divs (e.g. making them focusable
via the tabindex attribute), we'll no longer have the clean accessibility
tree we had before.

This commit handles description lists with expected stuff in their
accessibility tree, and should also prevent us from accidentally
including terms from a nested description list in the term count of
its parent description list.

See issue #248