The login screen is pretty custom full screen container and the standard
layout managers aren't really a good fit for the kind of layout that's
happening. This will be even more problematic with upcoming changes
to login banners, so we need to switch techniques.
This commit moves login dialog over to using a custom allocate handler
that has specific domain knowledge of the parts of the login screen
and where they go.
https://bugzilla.gnome.org/show_bug.cgi?id=703972
522fe8e8bf