Just move the minimal bits to this ClutterKeymapEvdev object. Much
of the functionality of a keymap is spread along ClutterSeatEvdev,
ClutterDeviceManagerEvdev and ClutterVirtualInputDevice. Future
refactors are due here.

Also, ideally keymaps are per-seat objects (at least keyboard state
is). We don't expose much info about seats altogether outside the
evdev device manager implementation. We just poke the main seat at
places, but eventually seats should be public.