Qt: move paymentACK handling to paymentserver

- add new slot handlePaymentACK() to paymentserver, which handles
  paymentACK messages (currently we just display them)
- make paymentACK message a modal information dialog
- change some QObject::tr() to just tr()
- clarify the processPaymentRequest() error, when IsDust()
- small string change to prevent a tripple + usage with QString