wacom: Show distinct entries for Wacom Express Key Remote

If you did not know the Wacom Express Key Remote (or EKR for short), let me introduce it to you:

This is a hand-sized standalone pad device, meant to be used together with drawing tablets, providing additional buttons and a touch sensitive ring that can be mapped to actions and keypresses.

These pads were so far handled in GNOME, but in a very subtle manner. As the EKR is implicitly paired to a tablet, it was possible to map EKR buttons from the paired tablet once the pad OSD UI from GNOME Shell was shown.

As this device basically just needs a "Map buttons" action to configure it, it just didn't sit well in the older Settings UI, it would get a separate page with just a lone button in there. So its support has been kind of an easter egg so far.

But the new UI can indeed accomodate better a device that is neither tablet nor stylus, and has few options. This commit adds the EKR as a separate AdwPreferencesGroup.