Missing GUI features

[pavel-kokolemin]

• [ ] New wallets should be scanned from the current tip, not genesis.
• [ ] Password should be prompted when creating a wallet.
• [ ] There should be an option to enable/disable saving mnemonic.
• [ ] Add command to show mnemonic.
• [ ] Write unit tests for GUI backend.
• [ ] Show QR codes for addresses.
• [ ] Allow changing wallet password.
• [ ] Missing features related to tokens (issuenewtoken, issuenewnft, sendtokenstoaddress).
• [ ] Show the list of won stakes (the same way we do with transactions).