fully qualified object locations

[Robert McQueen]

Can we add a bus type for bus names? If we have this we can establish
the convention that a struct of bus name and object path can be
demarshalled into a proxy objct. At the moment, a type 'o' can't always
correctly be assumed to refer to an object on the service that sent the
message. It could be sent alongside a string representing some other
service, and it's not safe to assume that's a bus name unless we have a
type that validates them.

Regards,
Rob