port the custom drag code to the qt4 way

- turn PagerWindowDrag into a simple namespace, and make the former c'tor a function to populate the QMimeData
- canDecode/decode: just use QMimeData, instead of use the drag events
... because of this, manually accept() the event on drop
- create a QDrag object and fill it with mime data, pixmap and hostspot information
- remove few unnecessary includes