Removed the necessity to append the detected object index (related to the frame) to the effect JSON - which makes the JSON smaller and the performance better.
Added "has_tracked_object" property on EffectBase so that the AddEffect function checks for this property instead of the Tracker effect name - this way it's possible to support other effects that have tracked objects but different names.
- Also removed "crop" from Clip class, as it was never implmeneted correctly, and we have a fully functional "crop" effect when needed
- Added caching to Clip class, to optimize previewing of cached frames (much faster than previous)
- 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();`
