Add 2 more buttons to XP-Pen Artist 12

[redstrate]

We emulate the touch bar as two additional buttons for this tablet.

See https://gitlab.freedesktop.org/libevdev/udev-hid-bpf/-/merge_requests/222

[redstrate]

For some reason KDE Plasma is showing 3 buttons, this is probably our bug but I should look into it.

[redstrate]

It also looks like the SVG shows the strip on there.

[whot]

just a fly-by comment: in the SVG we rely on certain IDs and classes for the components, see the (slightly hidden data/layouts/README.md).

Our test suite checks for required things to be present (if you have a Button A we expect a Button A, Button A Leader, etc.) but we don't check for those to be not present if not needed. IOW an SVG may have an element with id ButtonF when the tablet's data file doesn't actually have a button F. We rely on whatever renders the SVG to filter that - this may be where the issue is coming from?