Graph vis app: Get node positions when exporting GML

At the moment, there is no way to retrieve node positions dynamically generated by the layout (e.g. force layout, cose, etc) in Dash or by user dragging.

They are not stored in the Python graph object. They possibly can be retrieved from the JS using Cytoscape.js, however, this solution is not trivial.

Created a Plotly Community Forum post: https://community.plotly.com/t/dash-cytoscape-returning-node-positions-from-layout/23818/2