qt: Add Copy Address Action to Payment Requests

Currently, the only way to copy the address of a payment request is to double-click on the payment request and then click on the copy address button. This commit adds a convenient context menu action to copy the address of a payment request.

src/qt/receivecoinsdialog.h

@@ -69,6 +69,7 @@ private Q_SLOTS:
     void updateDisplayUnit();
     void showMenu(const QPoint &point);
     void copyURI();
+    void copyAddress();
     void copyLabel();
     void copyMessage();
     void copyAmount();