loginDialog: put "Not Listed?" button and user list in separate container

loginDialog: put "Not Listed?" button and user list in separate container

The user list and the "Not Listed?" button get shown and hidden at the
same time, so we can simplify the code by putting them in a new
subcontainer.

This commit creates a userSelectionBox container that both actors get
put in, and changes all the code that shows and hides these actors to
show and hide userSelectionBox instead.

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