Sets an icon to be used when an icon has not been set using set_icon() . The icon
passed in must be a GdkPixbuf object.
See set_icon() for a description of
what the icon is used for.
You might call this method if your application allows the icon for the
window to be changed at run time. Setting a default icon allows the
window to maintain a unique identity even if the icon set at run time
cannot be found or could not be loaded properly for some reason.
This method must be called statically.