Provide a way to know user timeformat (12h/24h) preference

In some countries, 12h and 24h time format both makes sense and are widely used according to personal preferences, it would be a good thing to have a central place to keep this setting (and thus avoid relying on some GSettings key that are not desktop-agnostic). It would also allow the Greeter (GDM/LightDM…) to know the preferred time format and thus show something more user-friendly across the desktops