ext-layer-shell: new protocol extension

The ext-layer-shell protocol can be used to allow clients to create
portable desktop components. It's been known for the last 3 years as
wlr-layer-shell [1]. It's now used by a number of clients and
compositors [2].

[1]: https://github.com/swaywm/wlr-protocols/blob/master/unstable/wlr-layer-shell-unstable-v1.xml
[2]: https://github.com/swaywm/wlr-protocols/wiki

Signed-off-by: Simon Ser <contact@emersion.fr>
3 jobs for layer-shell in 50 seconds (queued for 2 seconds)