- 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
This changes the type of the private `path` variable to QString
internally (which we were converting it to for a bunch of operations
anyway), and uses QString's more robust string-manipulation and
comparison methods to ensure that only filenames that _end in_ an
.svg/.svgz extension (case-insensitively) are recognized as SVG files.
Previously, a filename such as "Title.svg.png" would be interpreted as
an SVG file.
The change in QString type is purely internal, all of the class's
interfaces remain the same, and accept `std::string path`. We simply
convert it to/from QString at opposite places in the code, now.
`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();`
