Add hardware wallet support

[the-metalworker]

Some desktop wallets e.g. Electrum enable users to spend coins from their hardware wallets. Users open the wallet app, plug in their hardware wallet, then confirm each transaction on the hardware wallet to sign and broadcast it.

It would be nice to have this feature in zec-qt so I can use my hardware wallet with a wallet that is backed by my own full node. Right now all Zcash hardware wallet users have to give their addresses to a third party node run by the hardware wallet company. This is bad for privacy.

Even though this feature would only work with t-addresses today I think this is an improvement in privacy for hardware wallet users, and will get the wallet ready for whenever the hardware wallet companies add support for z-addresses too.

[BostonZcash]

A desktop wallet supporting Ledger is apparently critical to ever seeing Zondax's shielded app released: https://twitter.com/zondax/status/1414925545108819968

This would be a very welcome feature for Ledger zodlers for that reason alone.