unstable: add xdg-decoration protocol

This adds a new protocol to negotiate server-side rendering of window
decorations for xdg-toplevels. This allows compositors that want to draw
decorations themselves to send their preference to clients, and clients that
prefer server-side decorations to request them.

This is inspired by a protocol from KDE [1] which has been implemented in
KDE and Sway and was submitted for consideration in 2017 [2]. This patch
provides an updated protocol with those concerns taken into account.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Drew DeVault <sir@cmpwn.com>
Reviewed-by: David Edmundson <davidedmundson@kde.org>
Reviewed-by: Eike Hein <hein@kde.org>
Reviewed-by: Alan Griffiths <alan.griffiths@canonical.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>

[1] https://github.com/KDE/kwayland/blob/master/src/client/protocols/server-decoration.xml
[2] https://lists.freedesktop.org/archives/wayland-devel/2017-October/035564.html