expose sctk Frame in a WindowBuilder extension trait

[tinaun]

as part of #1967, and since libdecor-rs seems to be stalled, let users write their own decorations if they are not satisfied with the ones defined by FallbackFrame by providing an extension method on wayland platforms

[kchibisov]

A step to this direction was made https://github.com/rust-windowing/wayland-csd-frame. I'm not sure how we should handle the initialization though.

Also, I think all the API is available right now for you to implement the decorations on your own and simply set none as decorations in winit. There's an issue though that we'd need to expose when server side decorations are being used, so you won't end up drawing your own client ones...