Built-in icon sizes.
Icon sizes default to being inherited. Where they cannot be inherited, text size is the default.
All widgets which use
GtkIconSize set the normal-icons or
large-icons style classes correspondingly, and let themes
determine the actual size to be used with the
-gtk-icon-size CSS property.
Keep the size of the parent element
Size similar to text size
Large size, for example in an icon view
.g_type : UInt64
Returns the type id (GType) registered in GLib type system.