This PR is a refactor that makes `_ActionSheetButtonBackground` widgets no longer record their own `pressed` state, but instead receive this state from their parent. In this way, `_ActionSheetButtonBackground` becomes a stateless widget. The children states are duplicate because the parent has to keep track of the state for rendering dividers.
An obstacle with this change is that `_ActionSheetButtonBackground` needs an object that is persistent across rebuilds to provide to `Metadata.data`. Either it is kept as a stateful widget without any actual states, or it is made stateless and its `Element` as the object. After discussion, the first option is used.
`_ActionSheetSlideTarget` is renamed to `_SlideTarget` since the alert dialog will soon use this class as well.
This refactor shouldn't need additional tests. Still, one test is added for a behavior that I broke during development and found not covered by the unit tests then.
3303973d99