Hide chapters in Outline Unlimited

Allow us to hide chapters in Outline Unlimited (the same way that we can hide characters, worldbuilding elements, and specific traits).

When I’m working with the AI, I generally don’t want it to know about what will happen in future chapters, only what’s happened so far. So if I’m working on chapter 5, say, I’ll typically only want the AI to know about what’s happened in chapters 1 to 4, not what’s still to happen in chapters 6 to 35.