xinput: add set-prop command

xinput: add set-prop command

There's no reason to require the user to know the difference between
set-int-prop, set-float-prop and set-atom-prop, and to know the required
format for each integer property, since we can just ask
XGetDeviceProperty.

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