Add a nice screen asking for camera permission

[slava-sh]

See juliuscanute/qr_code_scanner#345 (comment)

To Reproduce
Steps to reproduce the behavior:

1. Open the app in the iOS simulator
2. An uncaught CameraException is thrown

Part of

• 🤳🥫 Scanning (tracker) #526

[slava-sh]

As mentioned in the linked issue, we could use something like https://pub.dev/packages/permission_handler to avoid displaying QRView when the permission is not granted. We could show a nice screen explaining how to grant the permission in that case.

[Piinks]

@teolemon I'd like to work on this alongside some of the graceful fail states I mentioned today. :)

[teolemon]

Little sister was startled by this today, just after onboarding.

[monsieurtanuki]

My remarks:

1. we don't need videos, do we? Perhaps removing videos from the list of requested permission will make the dialog less intrusive
2. this looks like a standard title, and I think we can put our own localized labels
3. how do other apps handle the camera permission dialog?
4. [Edit: added] During the onboarding we could add a comment "You can scan barcode with your camera; you'll see an authorization dialog for that"

[g123k]

Implemented in the app a few weeks ago