The license text.
This can be used to set a custom text for the license if it can't be set via [property@AboutWindow:license-type].
When set, [property@AboutWindow:license-type] will be set to `GTK_LICENSE_CUSTOM`.
The license text will be displayed on the Legal page, below the copyright information.
License text can contain Pango markup and links.
[[email protected]_legal_section] can be used to add license information for the application dependencies or other components.