Allow offline usage of "JoinMarketQt"

[BitcoinWukong]

Ideally, "JoinMarketQt" would cache the wallet balances and transactions locally, to allow offline usage.

When the wallet is offline, it shows an offline indicator in the status bar, and display the balances and transaction histories base on the local cache. While some of the functionalities, like "CoinJoin", are disabled when its offline.

Once the wallet is back online, it would sync the wallet status, update local cache, and re-enable all features.