Improve appearance of the update details list

The listbox was missing a separator after the last list. Instead
of adding that, change to the frame/scrollbar trick we're using
elsewhere to make lists grow a scrollbar only when needed.
Don't make Done button blue

It is a little much to make a ordinary close button of an
informative dialog blue. Just leave it gray.
Fix a typo

We were setting the hscrollbar_policy twice.