Signal that shell is ready

With phoc!265 (merged) we can signal that the shell is ready so the compositor can unblank the screen to avoid flicker (and do more fancy things in the future).

This is currently based on !886 (merged) since that needs phosh-private protocol version 6 as well but i could split that out in case the the phoc side of that moves faster than phoc!259 (merged) .

We want to add a proper Breaks: once the phoc version to break against is clear.