Import / exprot json for new node groups

Created by: Durman

I'm ready to reimplement serialization into JSON to include data of node groups. Also this version won't support import of monads therefore whenever exported tree includes monads old import version will be used. Also I see sence to consider approach for exporting data blocks (text, mesh). Previous step #3560

Here is how current structure of JSON looks like:

{"export_version": "0.10",
"framed_nodes": dict(),
"groups": dict(),
"nodes": dict(),
"update_lists": []}

I'm thinking to change it in the new version and make it looks more like structure of Blender bpy.data module.

{"export_version": "0.20",
"node_groups": {  # it can include more than one node tree (main node tree + node group trees)
    "My tree": {
        nodes: dict(),
        links: dict()}
"texts": {
    "My script": str()}
"meshes": {
    "Some mesh": dict()}}

With this format it will be easy to add new data blocks support if there will be an interest in it.