Draft: ext-layer-shell: new protocol

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 2.

Signed-off-by: Simon Ser contact@emersion.fr

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Right now this is just a copy of wlr-layer-shell with s/wlr/ext/. I'm sending a MR to get feedback. Here are the things I'd like to address:

• Reset version to 1
• Add destructor to global (https://github.com/swaywm/wlr-protocols/pull/87)
• The layer argument to get_layer_surface duplicates functionality wrt. the set_layer request
• Maybe use less "magic int values" and more enums
• Hard Problem: what to do with the namespace argument? It's (supposed to be) used by compositors to disambiguate z-index conflicts. Any ideas for better solutions?