backends/wayland: Port to xdg-decoration-v1

The server-decoration protocol is deprecated, and the clients should use the xdg-decoration-v1 protocol instead.

kwin will indicate that it wants server side decoration. If the host compositor insists on client-side decorations, the wayland backend will render no csd. However, this is de-facto state at the moment too, perhaps it can be improved by using libdecor?

Regardless, no csd is the current state too, except that both host and nested compositors can enter a loop where one side says to use CSD and the other side says that they want SSD.