This drops the AtkText implementation, and also strips handling
of children out. Instead of listening for enter/leave/press/released,
just listen for state changes on the widget.
At the same time, drop the AtkText implementation.
Also, reintroduce a GtkBoxAccessible, since the previous
way of setting the role in get_accessible was affecting
GtkBox subclasses negatively.
Adjust expected test output to match.