Creates a widget that can be used as a drag icon for the given
Supported types include strings, `GdkRGBA` and `GtkTextBuffer`. If GTK does not know how to create a widget for a given value, it will return null.
This method is used to set the default drag icon on drag'n'drop operations started by `GtkDragSource`, so you don't need to set a drag icon using this function there.
A new `GtkWidget` for displaying