Add Paper Wallet Generation and Printing Support to Bitcoin Core

Ported with permission from @andymeows dogecoin commit.

Adds support for printing paper wallets from the wallet.

Includes warning if connected to network.

Discussion on security can be viewed here: https://github.com/dogecoin/dogecoin/commit/4c5276d4ad3346c46eb9e938d79a6d7e2154f3e9#commitcomment-7841976