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

src/qt/bitcoingui.cpp

@@ -763,11 +763,6 @@ void BitcoinGUI::handlePaymentRequest(const SendCoinsRecipient& recipient)
     walletFrame->handlePaymentRequest(recipient);
 }
 
-void BitcoinGUI::showPaymentACK(const QString& msg)
-{
-    message(tr("Payment acknowledged"), GUIUtil::HtmlEscape(msg), CClientUIInterface::MODAL);
-}
-
 void BitcoinGUI::setEncryptionStatus(int status)
 {
     switch(status)