Creates a new GtkIconFactory. An icon factory manages a collection of icon sets; an icon set manages a set of variants of a particular icon. Icons in an icon factory are named by a stock ID, which is a simple string identifying the icon. Each GtkStyle has a list of icon factories derived from the current theme; those icon factories are consulted first when searching for an icon. If the theme doesn't set a particular icon, GTK+ looks for the icon in a list of default icon factories. Applications with icons should add a default icon factory with their icons, which will allow themes to override the icons for the application.