When the user closes an expanded set, such as Expert
Properties, any sets with that name should remain closed
for other property sheets and sets. Currently this works,
but the information is not persisted across IDE sessions.
Note - I would like to postpone implementing this to
possibly post-merge, in order to use the new Registry API,
rather than SystemOptions.
Implemented, along with storage of preferred sort order
and whether the description area is visible.