Restore backup should offer to scan a QR code

[firepol]

On the mobile app, I see the option to scan the QR code for the secret, but not for "Restore an existing backup wallet", e.g. I created a wallet from the desktop, it would be nice from the desktop app to be able to show that long json as a QR image to scan, and from the mobile app scan it to have restore the backup on mobile.

Copy pasting the backup to a phone, using email or other sharing services or whatever is not really secure. Also to backup from the phone should offer the option to scan a QR, so from a desktop PC (maybe airgapped, for extra security) with a webcam a user could scan the code from there and safely paste the backup code in a safe place.

Maybe this relates to #16 a bit.

[RichardAH]

I think there's just too much data in the typical backup to fit into a QR code. If you can propose an alternative to a QR that will contain the amount of data needed I'd consider it