gtk:drag-source-set (widget mask targets actions)

widget -- a gtk:widget object
mask -- a gdk:modifier-type bitmask of buttons that can start the drag
targets -- a list of target entries that the drag will support, may be nil
actions -- a gdk:drag-action bitmask of possible actions for a drag from this widget

Sets up a widget so that GTK will start a drag operation when the user clicks and drags on the widget. The widget must have a GDK window.