Polishing and fixing existing code

As GSoC is inevitably going to the end I started polishing and fixing existing code. Not all points of my initial plan was achieved but the aim for these last days is to prepare code so it can be extended to a full multi-account support in future.

Keeping roster items expanded

After implementing roster proxy model to serve multiple RosterModels I had to update RosterGroupExpandinessPersister class (that keeps given items expanded, also after Swift restart) to manage AccountRosterItems too.

Collapsed groups were stored in string list as a setting. I have implemented CollapsedRosterItemsSet where information about each account and its groups state is stored. We can get info whether item is expanded/collapsed with convenience methods. RosterGroupExpandinessPersister now uses this new set instead of string list. CollapsedRosterItemsSet is serializable so it's easy to load/save it.