userMenu: Offer "Switch Session" item on multi-session systems

Since commit 518282e1, we hide both "Switch User" and "Log out"
on single-user systems. However, if there is more than one session
available, users may still want to get back to the login manager
to change sessions.
Add both "Log out" and "Switch Session" items in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=675802