[RFC v2 libinput 0/2] Buttonset interface - numbered axes

[Peter Hutterer]

This is a re-vamped version of the buttonset interface. Still WIP but I'd
like to get some comments on the API.

Changes to the last version:
This version now uses numbered axes instead of typed axes. Previously the
interface used a unique axis type to deal with axes (e.g.
LIBINPUT_BUTTONSET_AXIS_RING_LEFT). This approach was ditched, instead a
device has a number of axes, each with a type. A caller can check how many
axes are there and the type for each axis. In the event interface, the axis
number is used, not the type.

A whole bunch of axes have been added too though the wacom pad interface
doesn't expose them, only wheel and ring.

Branch is available here:
https://github.com/whot/libinput/tree/wip/buttonbox-interface

It's still missing a couple of details, tests, checks, etc. but it should
show the interface well enough.

Cheers,
  Peter