Gets the data associated with a drag.
When the data is received or the retrieval fails, GTK+ will emit a
drag_data_received signal. Failure of the retrieval is indicated by the length field of the
selection_data signal parameter
being negative. However, when drag_get_data is called implicitely because the
gtk_dest_default_drop was set, then the widget will not receive notification of failed drops.
the widget that will receive the drag_data_received signal
the drag context
the target (form of the data) to retrieve