[RFC libinput] Add dial input device support

[PrasannaKumar Muralidharan]

> remember that libinput is a low-level library and doesn't know about how a
> specific device is being used by the layers above it. Hence a value in
> degrees isn't any less useful than a value in miscellaneous units which is
> what the _discrete() API bit is for. If the caller cares about units, it
> uses discrete steps, if it cares about number of rotations, it uses
> degrees.
>
> The only requirement in libinput is that you have a units -> physical
> degrees mapping which is something we used the hwdb for (in the case of
> mouse wheels anyway).

Got it.

Thanks,
PrasannaKumar