To change the flags and the pot and body variables after the gadget is
displayed, the program can call newmodifyprop().

    void NewModifyProp( struct Gadget *gadget, struct Window *window,
                        struct Requester *requester, unsigned long flags,
                        unsigned long horizPot, unsigned long vertPot,
                        unsigned long horizBody, unsigned long vertBody,
                        long numGad );

The gadget's internal state will be recalculated and the imagery will be
redisplayed to show the new state.  When numGads (in the prototype above)
is set to all ones, newmodifyprop() will only update those parts of the
imagery that have changed, which is much faster than removing the gadget,
changing values, adding the gadget back and refreshing its imagery.