Add auto-save

[AlexBxl]

Overview

I get why this is not done in other graph editors, with async execution of nodes etc you really don't want to auto save. But manual saving is not an expected behavior for web documents anymore, I lost a ton of work because I just closed/refreshed the browser.

Always save the state, if it needs some external execution, don't persist the execution!

When the user goes back to the document, prompt them if they want to load the current state or the saved state. If your graphs have no side effects just save it.

/rant I really hate this UI pattern for graph editors 😆