Add support for seed phrases

[the-metalworker]

Rather than save a file with my private keys, I would like to back up my wallet with a strong seed phrase instead. 24 words should be enough entropy for the foreseeable future and is standard in many wallets.

[adityapk00]

Thanks for filing this issue. There's a limitation in the zcashd RPC that prevents a wallet from getting the HD seed, so it's not implementable now. However, we're working with zcashd team to enable this soon