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
paymentserver: init all class members in constructor
Merge pull request #3146 from Diapolo/paymentserver_init

paymentserver: init all class members in constructor
Merge pull request #3130 from Diapolo/paymentACK_via_message

Qt: move paymentACK handling to paymentserver