New structure for JSON files and new approach of adding changes into the JSON format

Слиты nikitronn запросил слияние node_groups_to_json в master

Created by: Durman

Addressed problem description

This PR should make it easier to add new features into JSON format like saving mesh data blocks, text script files etc. Also it adds supporting serialization of node groups into JSON

closes #4096 (closed) #4100 (closed)

What's new:

  • JSON file format
  • Group tree export/import
  • Node presets for group nodes
  • Node presets inside group trees

Preflight checklist

  • Code changes complete.
  • Code documentation complete.
  • Manual testing done.
  • Unit-tests implemented.
  • Ready for merge.