loginDialog: (Optionally) show logo below user list

The optional logo on the login screen is currently shown in the
top bar, which is not only a rather unprominent position, it also
gives the wrong suggestion of a clickable element.
Newer designs call for the logo to be shown horizontally centered
at the bottom of the screen, so implement that instead.

https://bugzilla.gnome.org/show_bug.cgi?id=694912