ext-session-lock-v1: new protocol

This protocol allows for a privileged Wayland client to lock the
session and display arbitrary graphics while the session is locked.

The client is responsible for performing authentication and informing
the compositor when the session should be unlocked. If the client
dies while the session is locked the session remains locked, possibly
permanently depending on compositor policy.

Signed-off-by: Isaac Freund <mail@isaacfreund.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
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