[Qt] remove GUIUtil::getSaveFileName() default arguments

- harmonize function with GUIUtil::getOpenFileName()
- also make PNG Image singular (grammar)

src/qt/transactionview.cpp

@@ -273,7 +273,7 @@ void TransactionView::exportClicked()
     // CSV is currently the only supported format
     QString filename = GUIUtil::getSaveFileName(this,
         tr("Export Transaction History"), QString(),
-        tr("Comma separated file (*.csv)"));
+        tr("Comma separated file (*.csv)"), NULL);
 
     if (filename.isNull())
         return;