emit progress signals from QFileSystemModel::dropMimeData()

The copy and move operations performed by this can be slow and it would be nice to have some feedback for the user.

Emitting signals
void progressRangeChanged ( int minimum, int maximum )
void progressValueChanged ( int progressValue )
would facilitate connection to a QProgressDialog or similar.

The alternative is to intercept the event in the item view and replicate the functionality there, but I think this would make the class more useful. It could also benefit from a cancel slot.