Guidance on sharing state between components

[pieterdd]

It would be nice to have some guidance on best practices regarding state sharing between Relm4 components. These are the options I've considered so far:

• Introduce global state using SharedState. This is relatively easily to implement but may make it harder to write isolated automated tests. The tab_game example uses this approach.
• Pass down references to child components. I tried this at some point and got stuck trying to express the lifetime constraints.
• Perhaps passing down RefCell/RefMut variables could work. Haven't tried this yet. It may sidestep lifetime issues but could introduce runtime panics if not managed properly.
• Rely fully on message passing as opposed to shared state. This may be elaborate to maintain especially if there's a big hierarchy in your components.