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