Add mode=xi2 for using mtview as XI2 app

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Hook up XI2 backend

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Hook up pressure and mt touch minor/major

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Register for XI 2.2 in the main dpy

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Grab the device on startup

This is iffy with current servers, they won't actually give us the second
(simultaneous) touchpoint. Still, better than not grabbing it.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>