Add xdg-output protocol

This protocol aims at describing outputs in way which is more in line
with the concept of an output on desktop oriented systems.

Some information are more specific to the concept of an output for a
desktop oriented system and may not make sense in other applications,
such as IVI systems for example.

The goal is to gradually move the desktop specific concepts out of the
core wl_output protocol.

For now it just features the position and logical size which describe
the output position and size in the global compositor space.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>