This adds a virtual function to ClutterInputDevice to translate a
keycode from the hardware_keycode member of ClutterKeyEvent to an
evdev keycode. The function can fail so that input backends that don't
have a sensible way to translate to evdev keycodes can return FALSE.
There are implementations for evdev, wayland and X. The X
implementation assumes that the X server is using an evdev driver in
which case the hardware keycodes are the evdev codes plus 8.

Reviewed-by: Emmanuele Bassi <ebassi@linux.intel.com>