The current user status menu allow to set the session status,
which also influences the IM status when signed in with
mission-control. However, the way it is presented to the user
makes it hard to figure out how the statuses interact or that
there are two distinct status in the first place.
Therefore, use a separate control for each status, and update the
overall look to match gnome-contacts.
https://bugzilla.gnome.org/show_bug.cgi?id=652837
We haven't actually been calling the top-right menu "status menu" for
quite some time, so use the upcoming code changes as an excuse for
renaming it to "user menu".
