The Wacom Cintiq 24HD and later tablets send specific key events for
hardware/soft buttons. KEY_PROG1..KEY_PROG3 on earlier tablets,
KEY_CONTROLPANEL, KEY_ONSCREEN_DISPLAY, and KEY_BUTTONCONFIG on later tablets.
We ignore KEY_PROG1-3 because starting with kernel 5.4 older tablets will too
use the better-named #defines.

These differ from pad buttons as the key code in itself carries semantic
information, so we should pass them on as-is instead of mapping them to
meaningless 0-indexed buttons like we do on the other buttons.

So let's add a new event, LIBINPUT_EVENT_TABLET_PAD_KEY and the associated
functions to handle that case.

Pad keys have a fixed hw-defined semantic meaning and are thus not part of
a tablet mode group.

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