API for getting the current X domain

[keller-mark]

Use case: in react app, want to subscribe to X domain changes

Current workaround: I figured out how to do this using the underlying higlass API to subscribe to every viewConfig update, but this is not documented and relies on knowledge of the higlass view config

https://github.com/keller-mark/gosling-kpmp-example/blob/main/src/example/WidgetNavigation.js#L339

Possible solution: could offer a Gosling-specific JS API to subscribe to X domain changes