Attempt to reduce `Rc`s in the UI

[fengalin]

See https://kyren.github.io/2018/09/14/rustconf-talk.html as an inspiration.

This is what I plan on exploring:

• Remove Ref<Cell<>> for all controllers except MainController.
• Move shared states (such as boundaries: Rc<RefCell<ChaptersBoundaries>> or current_position: u64,) into a struct which is accessible from MainController.
• Only pass the Ref<Cell<MainController>> to callbacks.

Callbacks would be able to interact with any controller logic and state using only one Ref<Cell<>>. And shared states would no longer need to be Ref<Cell<>> or copied & synchronized.

[fengalin]

This is not an ECS-like approach since the UI should remain Object data oriented and not behaviour data oriented.