stock.id in the icon factory, returning an icon set
if found, otherwise NULL. For display to the user, you should
use gtkStyleLookupIconSet on the GtkStyle for the
widget that will display the icon, instead of using this
function directly, so that themes are taken into account.gtkIconFactoryLookup(object, stock.id)objectstock.idGtkIconSet] icon set of stock.id.