Add support for X Generic Extension events

With these patches, we are able to mark an XGE event as such and
generate the correct header for it.

XGE events can be found in the X Input Extension v2++.

Signed-off-by: Daniel Martin <consume.noise@gmail.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Peter Harris <pharris@opentext.com>