unlockDialog: Remove clutter constraints from the code

These cause annoying allocation cycle warnings, and it's simpler to
just express our desired layout in terms of nested containers.
Adapt the theme to match as well.

https://bugzilla.gnome.org/show_bug.cgi?id=706843
gdm: Remove constraints from authPrompt / loginDialog as well

https://bugzilla.gnome.org/show_bug.cgi?id=706843