FR: Event/callback on src/vol device assigned to zone by user

If a device can tell exactly which zone(s) it is attached to, then this would allow for me deeply integrated control, for eg, it would become much easier to implement stop on harmony activity change across multiple zones while ensuring only the correct zone is affected. Similar with resume play of last playing when attached to a zone and the correct activity is selected.

For eg, a callback on both source and volume devices to create a per-zone instance. Likewise a callback to indicate removed from a zone.

The above scenario can currently be done, be really only reliably if only a single zone is attached to the controls.

It is useful for volume as well as source, as perhaps depending on source selected I may choose to implement the volume extension differently, for eg on one source I map to relative up/down pressed on IR while on another I map to direct absolute volume level via a network API.