Add a high-level setting that gives us more freedom to tweak
font rendering knobs according to our needs. It has a 'manual'
value that lets users continue to influence font rendering using
the low-level font-related settings as before.

Once the schemas have this, we can support setting this session-wide.
See gsettings-desktop-schemas!79

The initial implementation of 'automatic' font rendering is fairly
simplistic: if the monitor dpi is less than 200, prefer sharpness,
so turn on metrics hinting and slight hinting. If the monitor dpi
is at least 200, we both off.