The license of the program, as a value of the %GtkLicense enumeration.
GtkAboutDialog will automatically fill out a standard disclaimer
and link the user to the appropriate online resource for the license text.
GTK_LICENSE_UNKNOWN is used, the link used will be the same
specified in the
GTK_LICENSE_CUSTOM is used, the current contents of the
GtkAboutDialog:license property are used.
For any other
GtkLicense value, the contents of the
GtkAboutDialog:license property are also set by this property as
a side effect.