Add compositor APIs for xdg decoration unstable v1

The xdg-decoration protocol lets the compositor tell clients that it doesn't want them to draw window decorations.

This is different from the env var QT_WAYLAND_DISABLE_WINDOWDECORATION, as the env var relies on the clients being executed correctly, and will only work with Qt clients, but xdg-decorations should work for clients written with other toolkits as well.