Ultimately, we want to add support for GDM's new ChoiceList
PAM extension. That extension allows PAM modules to present
a list of choices to the user. Before we can support that
extension, however, we need to have a list control in the
login-screen/unlock screen. This commit adds that control.
For the most part, it's a copy-and-paste of the gdm userlist,
but with less features. It lacks API specific to the users,
lacks the built in timed login indicator, etc. It does feature
a label heading.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1978>
