This needs more testing before merging
Right now for touch arbitration to work, we require the device group to be the same (i.e. they're hanging off the same physical bus). That's not always the case and statistically we have a lot more devices that have a built-in tablet and touchscreen than we have Intuos-like external tablets.
So let's default to the more common case - enabling arbitration with the first touchscreen/ext touchpad we find. If a subsequent device is "better", swap it out. Right now, the only heuristic we have here is the device group check but in the future we could get more precise.
Fixes #877 (closed), #690 (closed), #855 (closed)