Announce support for XI 2.0 to the server.

Technically, we're required to announce support for XI 2.0 through
XIQueryVersion. The behaviour of XI2 for clients that have not done so is
undefined, it just happens to work.

With XI 2.1 on the horizon, this may change so make sure we're clamining our
version correctly

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Julien Cristau <jcristau@debian.org>