Updates the icon to use @icon as the base background image.
If @icon is %NULL, @self will go back using style information
or default theming for its background image.
If this method is called and an icon name was already set as
background for the icon, @icon will be used, i.e. the last method
called between gtk_numerable_icon_set_background_gicon() and
gtk_numerable_icon_set_background_icon_name() has always priority.
Updates the icon to use @icon as the base background image. If @icon is %NULL, @self will go back using style information or default theming for its background image.
If this method is called and an icon name was already set as background for the icon, @icon will be used, i.e. the last method called between gtk_numerable_icon_set_background_gicon() and gtk_numerable_icon_set_background_icon_name() has always priority.