The license of the program. This string is displayed in a text view in a secondary dialog, therefore it is fine to use a long multi-paragraph text. Note that the text is only wrapped in the text view if the "wrap-license" property is set to true; otherwise the text itself must contain the intended linebreaks. When setting this property to a non-null value, the #GtkAboutDialog:license-type property is set to gtk.types.License.Custom as a side effect.
Set license property.