Add support for layout direction per graph

[LucaScheller]

Hey, I added a PR for https://github.com/jchanvfx/NodeGraphQt/issues/272 Not sure if these are all the spots where it needs to be fixed, seems to work for me though. Would be cool if we could merge this or sth similar into the master branch. Let me know if anything else needs to be fixed for this to work/if the changes are at the right spots or if I need to change var naming conventions! Cheers, Luca