the new destination window, obtained by gdk_drag_find_window().
the DND protocol in use, obtained by gdk_drag_find_window().
the x position of the pointer in root coordinates.
the y position of the pointer in root coordinates.
the suggested action.
the possible actions.
the timestamp for this operation.
FIXME
Updates the drag context when the pointer moves or the set of actions changes. This function is called by the drag source.