Re-enable Sonos support in mkchromecast

[xsdg]

Sonos support was implicitly disabled as part of the refactor in 40b3773 , because the way it was initially added harmed code readability, modularity, correctness, etc.

For anyone wanting Sonos support, you can use the stable branch, or check out a version of the codebase prior to that change.

The solution here is to finish refactoring cast.py (which has its own issues, even regarding Sonos), and then to re-incorporate Sonos support in a way that is sustainable for the codebase.

[psmgeelen]

@xsdg let me know if I can support :)