Move wl_shell into wlroots

This is just a trivial move that "works" e.g. mpv will create a wl_shell surface and play in it.

BUT there is a lot wrong with the code as it was. I'm getting segfaults when destroying xdg_shell, segfaults when destroying wl_shells, segfaults accessing freed elements in examples/compositor/wl_compositor.c.... (these aren't new, I'm getting them with the old code, but I can't test what I'm doing and I don't like it)

I have more commits that fix part of them, but I need more time to understand what makes xdg_shell xdg_shell and what makes wl_shell wl_shell and what I'm free to modify (e.g. changing how we store a list of surfaces for example) Unless we really can't move forward without this I'd rather just have another day or so to finish that