Turns this into a drag source for automatic DND.
Use [class@Gtk.GridView] instead
Calling this method sets `GtkIconView`reorderable to false.
| this |
a `GtkIconView` |
| start_button_mask |
Mask of allowed buttons to start drag |
| actions |
the bitmask of possible actions for a drag from this widget |
| formats |
the formats that the drag will support |