wpctl: add set-route subcommand

This MR adds a set-route subcommand to wpctl, similar to the set-profile subcommand. This fixes issue #488 (closed).

The ergonomics of this one are a bit weird, as can be seen in the example in this issue comment by @Alain, you don't just set a route on a sink, you need to set the route on the device the sink belongs to, passing a route index and the card.profile.device info property of the sink/source.

Potentially a set-route subcommand could also be implemented more high-level, in a way that gets the device id and card.profile.device value from just a sink/source and route index, but that also isn't how the rest of the wpctl subcommands work, so I implemented it the low-level plumbing way for now.

Let me know if you have any feedback to this.