Depending on the Boolen 'On', this either selects the gadget or deselects it.
This is for Gadgets that have their TOGGLESELECT flags set. The Gadget is
defined by its GadgetID number. If the Gadget is found in the Gadget list of
the window, its GADGDISABLED bit is either set or cleared. The Gadget is
redisplayed.