loginDialog: Scale focusBin instead of resizing it

When setting an explicit size as we do currently, rounding errors
(for instance introduced by padding not specified in pixels) may
affect the parent's size allocation, e.g. making it shrink or grow
each time the size is reset.
Rather than taking care of possible rounding errors, set up focusBin
to take up the available width and use scaling for the animation.

https://bugzilla.gnome.org/show_bug.cgi?id=675076