Save/Load layouted graph (backend)

[Shadow992]

We also want to save the layouted graph in backend. However this needs some MMT work. But at first TGView has to implement #38 to get an overview of dependencies and problems.

[Shadow992]

Ok finally I am quite sure what to save in backend and how. We should aim for a JSON file with the following structure:

{
	"nodes": 
	{
		[
			{
				"id": "id of node",
				"label [optional]": "text of node",
				"style [optional]": "style/class of node",
				"shape [optional]": "shape of node; neccessary for frontend",
				"mathml [optional]": "mathml if needed/wanted",
				"previewhtml [optional]": "HTML to show in node as preview",
				"url [optional]": "URL of node",
				"x [optional]": "x position of node",
				"y [optional]": "y position of node"
			},
			...
		]
	},
	"edges":
	{
		[
			{
				"id": "id of edge",
				"from": "Node to start edge from (source node)",
				"to": "Node to connect edge from (target node)",
				"label [optional]": "text of edge",
				"style [optional]": "style/class of edge",
				"weight [optional]": "Weight of edge",
				"url [optional]": "URL of edge"
			},
			...
		]
	},
	"cluster [optional]":
	{
		[
			{
				"id": "id of cluster",
				"nodeIds": ["id of node 1", "id of node 2", ...],
				"nodePositions": [{"x": "old x position of node 1 before clustering","y": "old y position of node 1 before clustering"}, ...],
				"label [optional]": "text for cluster",
				"type [optional]": "Type of cluster: lock nodes to region or create one new cluster node",
				"x [optional]": "x position of cluster",
				"y [optional]": "y position of cluster"
			},
			...
		]
	}
}

This structure is very similar to the current structure, which the backend delivers. Therefore it should be easy to extend the backend and frontend to produce/save/read this structure.

@Jazzpirate What do you think? Is this a good structure to start with?