xcb XI2: simplify and generalize tablet detection

We're looking for any tablet which has appropriate valuators, not
just Wacom tablets that have recognizable names. The name recognition
was always brittle and already stopped working, probably because of
changed udev rules or something like that.

Change-Id: Ib3a6dfdd9972b16f08f270e3f0a1aa54d6660c9f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>