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).