unstable: Add input-timestamps protocol

wl_pointer, wl_keyboard and wl_touch events currently use a 32-bit
timestamp with millisecond resolution. In some cases, notably latency
measurements, this resolution is too coarse to be useful.

This protocol provides additional high-resolution timestamps events,
which are emitted before the corresponding input event. Each timestamp
event contains a high-resolution, and ideally higher-accuracy, version
of the 'time' argument of the first subsequent supported input event.

Clients that care about high-resolution timestamps just need to keep
track of the last timestamp event they receive and associate it with the
next supported input event that arrives.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Acked-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>