WIP: Add close request to phosh-private protocol

I would have hoped to switch to

https://github.com/swaywm/wlr-protocols/blob/master/unstable/wlr-foreign-toplevel-management-unstable-v1.xml

but until !40 (closed) (or an even newer MR) is in lets do it that way. Once we have a newer wlroots version this part of the protocol can just go away but this way we don't block shell development.