Core: Backup Wallet

[samparsky]

Is your feature request related to a problem? Please describe.
Each wallet should be saved to its own file in local (or cloud) storage so it can be backed-up and restored independently of other user wallets.

You can find more details here
https://github.com/spacemeshos/app/wiki/Wallet-Data-File-Format

Describe the solution you'd like

1. Ensure proper backup of generated wallet file
2. Ensure the wallet file meets the stated specifications
3. Ensure your PR meets the contributions guidelines

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

See CONTRIBUTING.md for contribution guidelines