[RFC libinput] Add dial input device support

[PrasannaKumar Muralidharan]

> right. the approach we used for this was to have an API that exports the
> value in degrees and an API for get_value_discrete() or similarly named.
> Look up the libinput_event_pointer*axis* API to get an idea.

For several devices (example dimmers used for controlling lamps) value
in degrees is not useful. Exported value depends on device.

Regards,
PrasannaKumar