Create a new GTK2.IconSet. A GTK2.IconSet represents a single icon in
various sizes and widget states. It can provide a GDK2.Pixbuf for a given
size and state on request, and automatically caches some of the rendered
GDK2.Pixbuf objects.
Normally you would use GTK2.Widget->render_icon() instead of using
GTK2.IconSet directly. The one case where you'd use GTK2.IconSet is to
create application-specific icon sets to place in a GTK2.IconFactory.