Migrate several services/areas of extension API to a new model described in RFC 155 (tentatively Settings, Workspaces e.t.c)

Created by: twop

Steps:

introduce flat interfaces for API in both directions (ext -> main, main -> ext)
introduce single object that holds the entire worker state
migrate several simples services to the new model (tentatively Settings, Workspaces, Commands, Query transformer)
- main thread calls the new api object directly (no notion of a "services")
- rewrite/fix tests. That might be an interesting exercise to define our testing strategy for this rewrite
- replace/inline ExtClassX. Ideally we don't need one more extra layer of indirection here