gdbus: add a gdbus codegen extension

The extension can be used by projects which depend on libdex by passing
`--extension-path=path/to/dex-gdbus-codegen-extension.py` to
`gdbus-codegen`.

Every proxy creation then has an additional
`${interface}_proxy_new_future` function, which returns a future which
resolves to a proxy of that interface.

Every proxy method call then has an additional
`${interface}_call_${method}_future` function, which returns a future
which resolves to a boxed type that respresents the results of the
method call.

Additionally, Skeletons then inherit from DexDBusInterfaceSkeleton
instead of GDBusInterfaceSkeleton. It dispatches all the interface
method handlers, and the authorize signal in a fiber.

Depends on glib!4812 (merged)