[Qt] ensure payment request network matches client network

- replaces checks in SendCoinsDialog::handlePaymentRequest() that belong
  to PaymentServer (normal URIs are special cased, as only an isValid
  check is done on BTC addresses)
- prevents the client to handle payment requests that do not match the
  clients network and shows an error instead (mainly a problem with
  drag&drop payment requests onto the client window)
- includes some small comment changes also

src/qt/sendcoinsdialog.cpp

@@ -377,26 +377,8 @@ void SendCoinsDialog::pasteEntry(const SendCoinsRecipient &rv)
 
 bool SendCoinsDialog::handlePaymentRequest(const SendCoinsRecipient &rv)
 {
-    QString strSendCoins = tr("Send Coins");
-    if (rv.paymentRequest.IsInitialized()) {
-        // Expired payment request?
-        const payments::PaymentDetails& details = rv.paymentRequest.getDetails();
-        if (details.has_expires() && (int64_t)details.expires() < GetTime())
-        {
-            emit message(strSendCoins, tr("Payment request expired"),
-                CClientUIInterface::MSG_WARNING);
-            return false;
-        }
-    }
-    else {
-        CBitcoinAddress address(rv.address.toStdString());
-        if (!address.IsValid()) {
-            emit message(strSendCoins, tr("Invalid payment address %1").arg(rv.address),
-                CClientUIInterface::MSG_WARNING);
-            return false;
-        }
-    }
-
+    // Just paste the entry, all pre-checks
+    // are done in paymentserver.cpp.
     pasteEntry(rv);
     return true;
 }