ext-idle-notify: new protocol

This patch adds a new ext-idle-notify protocol. It allows clients to be
notified when the user is idle. Use-cases include e.g. power management
daemons.

This protocol is based on the org_kde_kwin_idle interface already being
used by KDE and wlroots compositors. The protocol has been sent to
wayland-protocols in 2015 [1]. This version renames and clarifies the
interfaces, and addresses the review comments.

[1]: https://lists.freedesktop.org/archives/wayland-devel/2015-December/026045.html



Signed-off-by: Simon Ser <contact@emersion.fr>
Closes: #8


Reviewed-by: David Edmundson <davidedmundson@kde.org>