Add foreign-toplevel-info protocol
This protocol provides the same information on toplevels as wlr-foreign-toplevel-management but does not allow the client to modify the state of said toplevels. This makes the protocol more suitable for use as a general way to address toplevels by protocol object, for example to integrate with the proposed ext-workspace protocol.
It is based on @ifreund's !59 (closed). I will follow up with a PR for the ext-workspace integration.