Improve QR code scanning on phones with high res cameras and slow CPUs

Instead of picking the largest available preview size, pick a size that balances resolution and decoding speed. Crop the preview to a square before decoding. All the test devices can now scan each other's QR codes.

The branch also improves tablet support by allowing the front-facing camera to be used if there's no back-facing camera, and adding a landscape layout for the QR code screen.

Could be backported.

Fixes #1068 (closed), improves #346 (closed).