Add support for udev XKB_FIXED_*
See wayland/weston!292 (closed)
We could add keymap hints to keyboard devices in the backends that compositors could choose to pick up (just like we have a map-to-output hint on pointer devices).
On DRM, we can populate it with udev's XKB_FIXED_*
properties. On Wayland/X11, maybe we can pick up the parent compositor's keymap attributes.