Start zbus::Proxy

Let's start somewhere. This provides a basic sync Proxy helper, with owned call/get/set methods. On top, a zbus_derive crate provides dbus_proxy procedural macro to ease implementation.

An upcoming generator will further help writing the trait from an introspection XML.

Left for further improvements:

• fixme/todo/improvements in proc-macro, better error handling etc
• unowned/callback based callers
• attributes to override the guessed method/properties naming
• signals
• async etc

Fixes #12 (closed).