Configure the account that pays the tx fees

[jasny]

LTO allows accounts to co-sign a transaction and sponsor the transaction fees.

If we require the user to use multiple accounts (Ethereum compatible for Ownables), they should be able to pick an account that has LTO to pay the tx fees, rather than having to transfer funds to each (sub) account.

See https://docs.ltonetwork.com/libraries/javascript/accounts#child-accounts