Split up clients/ into their own repository

As the $subj says, this issue is about taking out clients/ directory out of weston and move them into their own repository.

Would like to get some feedback on what other people think about that. Presumably this would be called wayland-clients, or wayland-demo-clients, somewhere along those lines.

simple-clients we currently have would probably make the gist of it, but probably also some of the demo clients. In weston, we'll just have desktop-shell client, ivi one, and the screenshooter (IDK maybe a few more?). I guess also that some the clients might need a bit of work to deal with compositors not implementing certain protocols, but I see it the other way around as well, people adding protocols in clients such that it supports those extensions.

One of the issue people pointed was that clients/ would depend on libweston, and that's a fact that we have today (well not directly obviously, but in the meson build part), so I had a stab at it trying build only simple-clients out of clients/ directory. See https://gitlab.freedesktop.org/mvlad/weston/-/commits/wip/mvlad/only-clients-build for an example. Note that was just a hack but it is technically possible if we decide this is not really helpful and potentially go on this route, which was the initial thing I tried.