- Parsing from string to Json::Value is now done by utility function
openshot::stringToJson() in Json.cpp, all SetJson() methods call it.
- Expand use of const member functions and args where appropriate.
- Use std::to_string() to format int/float values as strings.
- Correct mentions of nonexistent Json::JsonValue type in docstrings
`Json::Reader` has been deprecated for some time, so we replace it with
`Json::CharReader` generated by a `Json::CharReaderBuilder`, or (in the
one instance where we have a stream as input) `Json::parseFromStream();`
