gossip should save which groups have been collapsed within the roster between
sessions.

Other information:
If you're having a somewhat bigger roster you sure sort it by having a lot of
groups. Some of them might be of less interest than others so you most likely
will have some of them collapsed all of the time while others are shown.
It's really very annoying to have to collapse all less interesting groups after
each startup in order to get a useable view of the roster.