Add an API for restore/wake-from-sleep for an app

### What is the problem or limitation you are having?

A well behaved app should, where possible, save its current state when put into a hibernation state (either because of power loss or other hibernation event, such as being moved out of an active state on a mobile device).

### Describe the solution you'd like

A Toga app should be able to:

1. identify hibernation events from the platform
2. save the current app state
3. on restart, restore the previous app state

This means that widgets, windows, and the app itself should provide mechanisms to save their state.

### Describe alternatives you've considered

At the very least, it should be possible for users to implement their own hibernation/restore strategies. This means we should add the lifecycle events, and the mechanisms for adding user-space handlers for those lifecycle events. An initial implementation could be as simple as adding a public API for apps to declare their own manual restore behavior.

### Additional context

A fully fledged solution to this problem will run deep into an app's implementation. It will involve being able to reconstruct the app widget structure, restore any references to widgets, and restore any data sources.

Data sources are especially problematic, as they're an abstract API rather than a literal collection of values. If a data source is implemented as a proxy over an API, it may not be possible to literally "restore" the values, as there may not be an actual underlying data source. Therefore, a data source-backed widget can't just "save its values"; it will need to be able to restore its data source.

The save/restore functionality might also be useful for the niche "reset" use case described by #2317, so that should be kept in mind as part of API design.

No files, tests, or entry points are identified. Start by reviewing the existing app, widget, window, and data-source APIs alongside the reset use case in #2317; done would require a scoped lifecycle and user-handler design that addresses save and restore behavior.

Written by the indexing model from the issue text.