Enhance Json data handling

- 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

src/FFmpegReader.cpp

@@ -2424,14 +2424,14 @@ int64_t FFmpegReader::GetSmallestAudioFrame() {
 }
 
 // Generate JSON string of this object
-std::string FFmpegReader::Json() {
+std::string FFmpegReader::Json() const {
 
 	// Return formatted string
 	return JsonValue().toStyledString();
 }
 
-// Generate Json::JsonValue for this object
-Json::Value FFmpegReader::JsonValue() {
+// Generate Json::Value for this object
+Json::Value FFmpegReader::JsonValue() const {
 
 	// Create root json object
 	Json::Value root = ReaderBase::JsonValue(); // get parent properties
@@ -2443,23 +2443,11 @@ Json::Value FFmpegReader::JsonValue() {
 }
 
 // Load JSON string into this object
-void FFmpegReader::SetJson(std::string value) {
+void FFmpegReader::SetJson(const std::string value) {
 
 	// Parse JSON string into JSON objects
-	Json::Value root;
-	Json::CharReaderBuilder rbuilder;
-	Json::CharReader* reader(rbuilder.newCharReader());
-
-	std::string errors;
-	bool success = reader->parse(value.c_str(), value.c_str() + value.size(),
-	                 &root, &errors);
-	delete reader;
-
-	if (!success)
-		// Raise exception
-		throw InvalidJSON("JSON could not be parsed (or is invalid)");
-
 	try {
+		const Json::Value root = openshot::stringToJson(value);
 		// Set all values that match
 		SetJsonValue(root);
 	}
@@ -2469,8 +2457,8 @@ void FFmpegReader::SetJson(std::string value) {
 	}
 }
 
-// Load Json::JsonValue into this object
-void FFmpegReader::SetJsonValue(Json::Value root) {
+// Load Json::Value into this object
+void FFmpegReader::SetJsonValue(const Json::Value root) {
 
 	// Set parent data
 	ReaderBase::SetJsonValue(root);