clutter/x11: detect tablet type from the Wacom property
The xf86-input-wacom driver exports a property with the tool type as known by the driver. This is a more reliable choice than guessing based on the device name. In the touchscreen case, we simply use is_touch_device() to guess which one of the two options it is. Note that this code should never be hit anyway as we would've succeeded earlier with a previous is_touch_device() call.