[RFC libinput] Add dial input device support

[PrasannaKumar Muralidharan]

Hi Andreas,

> Instead of car steering wheels I would rather mention rotaty knobs that are
> common in car infotainment  systems to navigate through menues. Those are
> relative - you never get absolute angles just rotation ticks.
> But is that the type of device you had in mind? Those devices usually have a
> bunch of further buttons.. so buttonset sounds like a good generalization
> for those too.

Yes buttonset interface suits well for dial.

As of now kernel provides REL_DIAL. As of now I am thinking about
rotatory knobs or something like PowerMate device. Other use case is
dimmers used for light / lamps which return absolute value, has some
maximum value. I did not look much to find absolute dial interface
from kernel, so skipping it for now. I guess ABS_WHEEL Is not suitable
for this type of devices.

Regards,
PrasannaKumar