loginDialog: use user widget when doing verification

loginDialog: use user widget when doing verification

Right now, when a user item is clicked we remove all other users from
the list and position the item in the appropriate place on screen.

Ultimately, we're going to want to crossfade from the fully populated
list to the user prompt.  Since we're going to need to show the user
avatar in two different positions we can't simply move it.

This commit leaves the user item for the user list, and instead shows
a UserWidget actor during user verification, in the same way the
unlock dialog shows a UserWidget actor during reauthentication.

https://bugzilla.gnome.org/show_bug.cgi?id=694062