Support parsing json, which is escaped as a string in json value

[rualark]

Example:

{
  "a": "{\"c\": \"some data\", \"d\": 3}",
  "b": 2
}

This can be recursive (unescape json string and get json, which can also contain a string, which needs to be unescaped. Probably, it will be convenient to add button to such string which can be unescaped.

[lahmatiy]

I think we may add presentation for a string value as json like we do with as text.

However, it has open questions: how cheap to check that the string is a JSON value, is it possible to apply nested struct view for it etc