Game needs saving when the result is added

E.g. if you resign, the game wants to be saved to reflect that
resignation

https://bugzilla.gnome.org/show_bug.cgi?id=702157

src/gnome-chess.vala

@@ -878,6 +878,8 @@ public class Application : Gtk.Application
         undo_menu.sensitive = undo_button.sensitive = false;
         claim_draw_menu.sensitive = false;
 
+        game_needs_saving = true;
+
         if (opponent_engine != null)
             opponent_engine.stop ();