Let users be able to set dragCursor for QAbstractItemView

Some users report that the default cursors of QAbstractItemView during drag and drop (by startDrag()) is not satisfactory for being too small/undistinguishable. Then maybe it is helpful to add a public API to QAbstractItemView so that the automatically generated internal QDrag can be at least modified in terms of appearance.

Update: a clip of screen recording is uploaded. The cursor during dragging inside MainWindow is considered 'too small' by the original reporter. But either the issue is subjective or not, it's true that QAbstractItemView does not expose its QDrag.