Add GraphML export for manual positioning

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Actually exporting to GraphML should be implemented in graphp/graph#3.

This ticket serves both as a valid use case for its implementation and as a reminder to add the necessary glue code to take advantage of it within this project.

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GraphML format has been implemented: https://github.com/graphp/graphml

However, the export currently does not contain any attributes. We should look into this:
graphp/graphml#5

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As much as I'd love to work on this myself, there are currently no immediate plans to build this from my end (no demand at the moment and more important outstanding issues currently). If you need this for a commercial project and you want to help sponsor this feature, feel free to reach out and I'm happy to take a look.