Add general purpose node graph editor

[kraifpatrik]

We'd like to implement a general purpose node graph editor, where you connect nodes via their input and output slots. Just the UI - evaluation of the graph is up to the user and their specific use case! For inspiration on what such editor could look like, use programs like Blender or UE5/CRYENGINE/Unity etc.

Some basics for this are already laid out in FORMS_NodeGraph.gml (all marked as @private for now, so they don't appear in docs).