This interface handles the buttons on the physical tablet itself, including
the touch ring and the strip.

A notable difference to other libinput interfaces here is that we do not use
linux/input.h event codes for buttons. Instead, the buttons are merely
numbered sequentially, starting at button 1. This means:
* the API is different, instead of get_button() we have get_button_number() to
  drive the point home
* there is no seat button count. pads are inherently different devices and
  compositors should treat them as such. The seat button count makes sense
  when you want to know how many devices have BTN_LEFT down, but it makes no
  sense for buttons where all the semantics are handled by the compositor
  anyway.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Carlos Garnacho <carlosg@gnome.org>