Implement sub-graphs

[leissler]

The feature of sub-graphs is such a common feature for the better organization of graphs in many graph-based tools, that it could be a standard feature of such a system. Probably real subgraphs are more than just implementing another utility like the GroupComment node (but I'm not sure). Subgraphs have their own "ports" that interface the subgraph and look to the parent class like a node.

[Doppelkeks]

This will probably require some heavy lifting and rewrites especially with the targeted WYSIWYG flow. Since I'm targeting 1:1 data management with this framework, I'm not entirely sure if this is even implementable but I did not have the time to think it through, so it might very well be achievable.

I'm happy if someone wants to give this a shot or provide a concrete concept/architecture to talk through. :)