Prototype a Slimbus based backend

[PolyMeilex]

It seems that people hate the dep tree required to run zbus (with tokio or async-io), perhaps we could try to implement minimal backend with my experimental https://github.com/PolyMeilex/slimbus crate, which is zbus with nearly everything fancy stripped out.

Obviously I don't intend to mantain zbus alternative, the idea is that for such a simple usecase as rfd it can just stay in a frozen state and never be updated. Dbus spec will probably not rug pull us any time soon.