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.

I plan to open a followup PR introducing an ext-foreign-toplevel-mangement protocol to provide the other half of the functionality of wlr-foreign-toplevel-management shortly.