Add "watch only" wallets

[noxonsu]

https://github.com/swaponline/MultiCurrencyWallet/issues/2419

Not only that. We should be able to add "watch only" wallets. This is a supercool feature. If you have a cold wallet, why not add its balance to your browser, and watch how payments come in?

We can make even more sense to use "watch only" wallets - we can track invoices. Right now you create a separate wallet for every payment - its the best way to track your buyers, and invoices. This is great, but this means that you need to manually create new wallets, and eventually they grow to some crazy number of addresses hard to group or keep track of.