[rpc] [addresses] [qt] Add BTC / ZCL address viewing utility [$30]

[ch4ot1c]

Users should be able to use an RPC method to 'view pre-fork addresses' - BTC and/or ZCL addresses that correspond to a set of BTCP addresses.

These would of course fundamentally correspond to private keys, which a user may or may not own; we should therefore make it clear that this is a view-only utility option, and never touches or uses their loaded private keys at all.

This could then (possibly) be made into a GUI (Qt) component. Additionally, they would be able to view each chains' UTXOs from an explorer link.

Warning - we need to make any GUI component like this clearly described and separated, so it cannot be a danger to simple people trying to simply send BTCP.

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There is a $30 open bounty on this issue. Add to the bounty at Bountysource.

[ch4ot1c]

This also could be written up as a teaching aid, to help people understand base58check addresses

[michaelotis]

getlegacyaddress [btcpaddress] -- Should automatically decode and re-encode base58 to both BTC and ZCL with labels to distinguish or use a switch to output just one