Qt: move paymentACK handling to paymentserver

[Diapolo]

• 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

[laanwj]

Haven't looked at it in detail yet, but moving the message to PaymentServer makes sense as it only re-emits a message.

[laanwj]

I don't think we should make the paymentack message a tray message.
As we are not storing it anywhere for later reference at the moment (or are we?), it's good to give the user some time to read it or even copy it.

[Diapolo]

@laanwj That's fine with me... perhaps there are better methods for handling this message, but that's out of the score of this pr.

[Diapolo]

Updated:

• make paymentACK message a modal information dialog

[laanwj]

Hm I'd suggest to keep the signal, and subscribe to it locally; what if we want to do something else with the ACK later on?

[Diapolo]

I'm going to change that :).

[Diapolo]

Updated to reflect @laanwj's suggestion:

• add new slot handlePaymentACK() to paymentserver, which handles paymentACK messages (currently we just display them)

[laanwj]

ACK

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