Feature: Add Native Bitcoin Wallet (linked to BTS private key)

So this would lay the foundation for atomic swaps between bitcoin and bitshares, and would open bitshares up to a greater community.

Every BitShares Account has a bitcoin address attached via its private key.

* extract the bitcoin address from the active private key
* add a Bitcoin wallet tab - with basic data send&receive
* add API access via https://api.greenaddress.it/index.html (or via witnesses later on)
* add an easy BTC <> OPEN.BTC/blocktrades exchange (optional)

Possible problems:
Password changes require key changes, which would erase your access to bitcoin. this needs to be addressed.

No source files, tests, or entry points are named. Start by mapping the wallet UI and account/private-key handling, then define the scope for address extraction, send/receive, API access, exchange support, and the password-change risk; done requires an agreed design and implementation plan.

Written by the indexing model from the issue text.