add a warning if user keeps dangerous amounts of coins in the wallet

[mackuba]

I think it would make sense to warn the user that it's not a good idea if they keep e.g. 50 BTC in a desktop hot wallet. The question is what amount to set as the limit... I guess it would have to be specified in a fiat currency like US dollars, since BTC exchange rate can change by an order of magnitude in a few months, and it would be annoying to have to keep updating that.