Handle pad mode/group on X11
clutter_input_device_is_mode_switch_button
and clutter_input_device_get_mode_switch_button_group
are left unimplemented on X11. Similarly, events report mode/group=0 all the time.
Admittedly this could be much better after we move input devices to src/backends, but I did a couple of patches that implement missing mode/group support on X11 by poking libwacom.