Feature Request: Provide a way for clients to know when a watched file has changed

Cloud Code is implementing "interactive watch mode" giving the user control over when to trigger a skaffold session. This takes the form of a button in the UI. It would be great if it could indicate to the user that something changed (mark it with an icon / enable it etc.) to help draw attention and visibly mark the state as dirty.

**Feature request**

Skaffold provides a way (e.g. an event through its API) that indicates that a file or files that Skaffold is watching has changed. It might be nice to know _which_ files have changed, but the key part is just knowing when something changed.