Consider Rectangular and Image nodes as the same kind of node

[gcoutable]

Remove GQLNode#type in ReactFlow and don't fetch this info in the GraphQL fragment nodeFragment

Feature parity between rectangular and image nodes

Combine both RectangularNode and ImageNode into a FreeFormNode in the frontend. In a similar fashion, we should thus have only a FreeFormNodeLayoutHandler. We may change the GraphQL API / Programmatic API later but this is not an issue for now. It will allow image nodes to have children and support the drop of semantic elements. The image could be connected to the node by using CSS background property.

See:

• background-image: url("../img/blog.png") no-repeat center center;
• background-size: contain;
• background-repeat: no-repeat;

Test with an image node with child nodes to see the result