Add a hash or unique id column to VisualN fields

Ideally each drawing on the system should be uniquely identifiable and accessible. That would allow to share individual drawings via iframes and also make it easier for drawing fetchers to get specific drawings from multiple value fields even if the order of deltas is changed. The fetcher and resource providers would also benefit from having a hash (or unique id) column for their fields.
It reflects the concept of entity as a structured data storage each element of which can be used separately in different contexts as opposed to entity as an integral piece of content or data.