Add Paper Wallet Generation from Bitcoin Core

Ported by @andymeows and @whitj00 from @andymeows' dogecoin implementation.

Tested on Mac, Windows, and Linux with qt4 and qt5

Includes warning if computer is connected to Internet.

While not optimal, it is better than using a website like brainwallet.org and anything that makes cold storage easier for the average user is good IMO.

This is likely controversial and I am open to discussion on how to improve this.