Rules for Tools/SystemGadgets()
SystemGadgets(struct window *window, long class,
struct Gadget *gadget, long code)
To handle the system gadgets, the library has a routine,
`SystemGadgets()', that you call every time you receive an
intuition message. Pass it the window, `message->Class', gadget
pointer (if the message points to one,) and `message->Code'.
It will process the message and return a new Class value, usually
unchanged if this was not a system gadget call. You can then
procede as appropriate.