The license of the program, as free-form text.
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 [[email protected]:license-type] property is set to `GTK_LICENSE_CUSTOM` as a side effect.
The text may contain links in this format `http://www.some.place/` and email references in the form `mailto:[email protected]`, and these will be converted into clickable links.