The first two commits from !910. Since that seems to be blocked by design concerns (and dealing with xkb...). I think this should be fairly uncontroversial.
This should avoid issues where this setting is changed just by opening Gnome Control Center, as reported in #918.
This should also mean someone can tweak the settings as desired outside Gnome Control Center, and not have them clobbered unless they open the dialog in g-c-c.