Ledger hardware wallet support for Monero (XMR)

[armujahid]

1. I think it would be nice to have ledger support for XMR. Currently monero-gui has this feature already.
2. Another nice to have feature is to request to plug in the device only when sending transaction. Hardware wallet: request to plug in the device only when sending transaction monero-project/monero-gui#2952 that feature doesn't even exist in monero-gui. This feature should be optional because of security concerns (Some people might not want to store their view key locally)

[tuxpizza]

This is currently on-track, and will be coming next after we release support for Litecoin on Ledger.