Registers a built-in icon for icon theme lookups. The idea of build-in
icons is to allow an application or library that uses themed icons to
function requiring files to be present in the file system. For instance,
the default images for all of GTK2+'s stock icons are registered as built-in
icons.
In general, if you use add_builtin_icon() you should also install the icon
in the icon theme, so that the icon is generally available.