Empathy user status icons

As discussed in xdg-list [1], there's an icon missing for
"user-unknown" (and possibly "user-error") for gnome-icon-theme to be
fully matching with Empathy, as it is now part of GNOME.

Also, unless it's there a good reason for keeping the "user-invisible"
name, perhaps a rename to "user-hidden" would be desirable, since
"hidden" is the term used in Telepathy.

In my opinion, "invisible" is ambiguous since (in XMPP) you can cancel
a subscription so that your status is no longer visible to someone,
which is different from being hidden for someone (in the first case
the contact will see you as "user-unknown", in the second case they
will see you as "user-offline").

[1]: http://lists.freedesktop.org/archives/xdg/2009-June/010517.html

--
Fernando