[RFC libinput] Import buttonset interface

[PrasannaKumar Muralidharan]

> simply said, I don't know. It's one of the things where we have to write
> most of the interface, see if it makes sense and then remove the bits that
> we don't actually need. your main target right now is a rotary encoder
> but I do encourage you to think of how to deal with other devices that are
> generic devices (tv remotes, maybe a 3d mouse, dj mixers, etc.) and
> incorporate them. an interface just suitable for a rotary encoder won't get
> merged.

Sure I will think and come up with a patch.