Modelled to be mostly compatible to the xf86-input-wacom driver behavior. The
pad gets 7 axes, the first three of which are mute and the others are always
available but obviously only send events when the axis is there.

The strip axes are incompatible, the wacom driver merely forwards the device
events (which are a bitshifted value), libinput normalizes it and we just
expand this back into an integer range. Let's see how we go with this.

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