xdg-output: Add name and description events

This adds two events to the protocol. The goal is to allow clients to
give the user the ability to select outputs with the same names the
compositor uses and to identify outputs consistently across sessions.
The output name is a short and stiff identifier with strict limits on
permitted characters, which is suitable for storing in config files,
command line arguments, etc. A warmer "description" event is also
provided to (optionally) provide a more human readable name, and has
much broader restrictions on its form.

Signed-off-by: Drew DeVault <sir@cmpwn.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
[Jonas: Fixed formatting and commit subject]
Signed-off-by: Jonas Ådahl <jadahl@gmail.com>