matplotlib plotting: return Graph artist instead of Axes

[iosonofabio]

See #655 - this would improve consistency with the Cairo backend and also help users, however it breaks API so let's leave it for 0.11.

Once changed, docs need to be adapted as well, especially the visual style tutorial in the gallery.

Thanks @szhorvat for prompting the issue.

[ntamas]

Linking #655 here

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.

[ntamas]

This is already done in the dev branch, right? I've just tested it and it already returns a GraphArtist for me. Can this issue be closed?