protocol: add the wl_pointer.axis_value120 events

These events are for high-resolution wheels. Each logical wheel click
represents an axis value120 value of 120, fractions of a wheel click an
integer fraction thereof. The new event replaces
wl_pointer.axis_discrete for wl_pointer versions 8 and above.

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