Refactor CheckoutCard getState

[danimoh]

As the CheckoutServerApi.getState request does not depend on the currency type anymore (see #412), it does not make sense anymore that the CheckoutCards for every coin separately fetch and update the state. Instead, the Checkout view can take care of this and forward the updated state to the CheckoutCards. Same for syncing to server time.

Also, currently it doesn't make too much sense that the overpayment / underpayment logic is in the CheckoutCard base class as its only used for BTC and ETH. As this would change with #415 though, this should already be taken into consideration when refactoring the state update.