Add QR code scanning API

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

Toga has a camera API, implemented on iOS, Android and macOS. However, this interface is focussed on taking photographs, using an external camera panel/app.

Another common use case for cameras is to scan QR codes. Toga should have an API for capturing QR codes.

### Describe the solution you'd like

An API, similar to `await app.camera.take_photo` to capture QR codes. `await app.camera.capture_qr_code` would be an obvious starting point.

Calling the API would display a full-page live camera view (with a QR code "target" square); when a QR code is seen, the window would be dismissed, and the coroutine return the captured QR code.

As part of implementing this, it may be necessary to add a "live camera view" widget. That widget would allow the user to define a box that will display a view of the live camera. That widget should then have a `async def take_photo()` API, and handler for `on_qr_code()` that is invoked whenever a QR code is "seen". If a `on_qr_code` handler is provided, the camera view would be displayed with a target.

### Describe alternatives you've considered

Leave QR code implementations to the user to implement.

### Additional context

User @ezzygarmyz has implemented a QR scanner for Android in [this project](https://github.com/ezzygarmyz/QRScanner-android). This demonstrates how Android QR code APIs can be invoked to capture

iOS/macOS implementations will need to use AVCaptureSession APIs; there are many examples of usage online (e.g., [this one](https://www.programmersought.com/article/8211808189/) and [this one](https://github.com/mercari/QRScanner))

Start by reading the existing app.camera.take_photo API and its camera implementations on Android, iOS, and macOS. Compare the proposed app.camera.capture_qr_code entry point with the live camera view and on_qr_code alternatives, then clarify the cross-platform scope and behavior before implementation; done means an agreed API and working QR capture on the supported platforms.

Written by the indexing model from the issue text.