The nicklist is back! We now have a NickModel that can be attached to an IrcChannel

The nicklist is back! We now have a NickModel that can be attached to an IrcChannel
and that provides access to the nicks in that channel. We also have a NickListWidget, that
encapsulates a stack of NickViews (which are not-yet-fancy QTreeViews).
There are still some things missing, nicks are not sorted yet and the output will be improved
as well. But we can see nicks again, yay ;-)