Move init of RosterGroups collection and presenceHandler registration out of
the controlbox and rosterview plugins and into core, since this code is
generally useful (indeed necessary) even without those plugins.
We already emit an event when the roster is fetched from the XMPP server,
similarly, it would be useful to know when the roster was instead fetched from
the cache.
The filter was being hidden if the roster was not big enough to require a
scrollbar, but the roster might not be big enough because the roster is
actually being applied.
