Add version numbers to the backups and implement an import migration path for older backups

[jojobyte]

Error

I get this error after loading a wallet backup:

utils.js:1062 Uncaught (in promise) TypeError: Cannot read properties of undefined (reading '0')
    at getAddressIndexFromUsage (utils.js:1062:32)
    at getAccountWallet (wallet.js:1144:29)
    at async main (main.js:622:14)
getAddressIndexFromUsage	@	utils.js:1062
getAccountWallet	@	wallet.js:1144
await in getAccountWallet (async)		
(anonymous)	@	main.js:1150

As you can see, my screen is blank, so I can't test yet.

Originally posted by @riongull in #41

Diagnosis

This appears to be related to using an older wallet backup that was created before we added change address support.

We likely need to add version numbers to the backups and have some form of import migration path for older backups.