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

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

src/qt/addressbookpage.cpp

@@ -263,10 +263,9 @@ void AddressBookPage::done(int retval)
 void AddressBookPage::on_exportButton_clicked()
 {
     // CSV is currently the only supported format
-    QString filename = GUIUtil::getSaveFileName(
-            this,
-            tr("Export Address List"), QString(),
-            tr("Comma separated file (*.csv)"));
+    QString filename = GUIUtil::getSaveFileName(this,
+        tr("Export Address List"), QString(),
+        tr("Comma separated file (*.csv)"), NULL);
 
     if (filename.isNull()) return;