Since the caps-lock warning adds a lot of spacing to dialogs and the
lock screen, hide it by default and only show it when necessary. To make
the transition smooth instead of just showing the label, animate it in
using the height and opacity.
Also add some bottom padding to the label so we can show or hide that
padding, too.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/952
