staging/inputfd: Direct input access protocol

This is an updated version of v3 of the inputfd protocol draft by
Peter Hutterer <peter.hutterer at who-t.net>.

For previous versions, see:
v1: https://lists.freedesktop.org/archives/wayland-devel/2017-March/033626.html
v2: https://lists.freedesktop.org/archives/wayland-devel/2017-April/033819.html
v3: https://lists.freedesktop.org/archives/wayland-devel/2017-August/034828.html

Quoting the last version:
> This update is closer to v1 again than v2. Biggest change is: instead of a
> get_seat we have a get_seat_evdev request. Anything evdev is then hanging
> off that seat, basically in how it was before. The hierarchy of objects is
> thus something like this:
>
>     manager
>       seat_evdev
>         device_evdev
>         device_evdev
>         device_evdev
>
> and in the future we could have other interfaces like this:
>
>     manager
>       seat_evdev
>         device_evdev
>         device_evdev
>         device_evdev
>         device_evdev
>       seat_hidraw
>         device_hidraw
>         device_hidraw
>         device_hidraw
>       seat_w3c
>         device_w3c
>         device_w3c
>
> Having it split at the seat level means we don't have to pollute each
> interface with the different needs of the other interfaces. evdev is the
> easiest case, so that's done first. I dropped the extra properties, Bastien
> and I discussed this at GUADEC and we agreed it's better to implement the
> minimal version first and then see what we need before trying to come up
> with a generic solution.
>
> Cheers,
>    Peter

Signed-off-by: Robert Mader <robert.mader@posteo.de>