Add libdecor

[RonaldZielaznicki]

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Alright, this PR doesn't just add libdecor.

It also:

• renames wayland-generated/ to wayland-protocols
• renames include/ to /wayland
• and updates the readme to reflect these changes

Why?

A desire to push this repository's structure to be more in line with the other [thing]-headers repositories which hold multiple libraries under one umbrella. It'll not be hard to remove those changes if they are not wanted. I purposefully put them in a separate commit just in case.

[emidoots]

Excellent work, thank you! I verified the authenticity of the headers as well.