Add gtk_settings_reset_property

This api undoes an application-specific override and makes the
setting follow the system-wide values again.

https://bugzilla.gnome.org/show_bug.cgi?id=755008

gtk/gtksettings.h

@@ -140,6 +140,9 @@ void     gtk_settings_set_double_property  (GtkSettings            *settings,
                                             gdouble                 v_double,
                                             const gchar            *origin);
 
+GDK_AVAILABLE_IN_3_20
+void     gtk_settings_reset_property       (GtkSettings            *settings,
+                                            const gchar            *name);
 
 G_END_DECLS