Draft: Add support for tablet pad relative dials

This adds support for tablet pad relative dials (Wayland only). These work basically like mouse wheels (i.e. deltas only, no source information, 120-based values) and are attached on some of the Huion pads, e.g. Huion Inspiroy Dial Q620M. The actual implementation is a lot of copy/paste from the ring and strip support with s/ring/dial/.

Needed for this is:

• libinput!967 (merged)
• wayland-protocols!298
• libwacom!665 (merged)
• for testing: wev (wip/tablet-rel-dial
• for testing: gtk!7242

For local testing without a device that has a dial:

• install this file as /etc/udev/hwdb.d/99-rel-dial.hwdb: 99-rel-dial.hwdb and run sudo systemd-hwdb update. This assigns the udev properties and a libinput device group that is shared by the below libinput recordings.
• install this .tablet file as /etc/libwacom/99-rel-dial.tablet: rel-dial.tablet
• run sudo libinput replay tablet-pen.yml with this file: tablet-pen.yml Leave it running in the background.
• run sudo libinput replay rel-dial.yml with this file: rel-dial.yml Hit enter to simulate a few dial events.

Both libinput replay need to run at the same time but for events only the rel-dial.yml file matters. Without the tablet-pen.yml mutter won't see a paired tablet tool and won't set the input focus for the dial device.