[#34041] JToolbar Popup button modal icon class not customizable

### How to test

1. Access the Backend Banner Manager 
2. Access the "Trackes" view
3. See the icon bevor the "Export" Button (icon-cog)
4. apply patch
5. see the download icon (icon-download)

### Why change export --> download

We have no icon-export see: http://www.nonr.nl/jui/index.php/icons

But a download icon (icon-download)

### Tracker

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=34041&start=0