todos
The `Widget` trait in `iced_pure` needed to change a bit to make the implementation of `Element::map` possible. Specifically, the `children` method has been split into `diff` and `children_state`.
Scrollable
iced_pure
Button
... and reuse it in `iced_pure`!
Application
iced_virtual
`virtual` is a reserved keyword in Rust 😬
