login-dialog: Increase top/bottom padding

The login dialog has a maximum height set to prevent the user list
from growing indefinitively. However this approach fails if the
monitor height is smaller as said maximum, so add some additional
top/bottom padding to make sure there's some whitespace above/below
the user list in that case as well.

https://bugzilla.gnome.org/show_bug.cgi?id=685851