Abstract checkpoint saving to its own function

This also makes the save failed textbox not appear in special modes,
and allows custom levels to quicksave from checkpoints as well.

desktop_version/src/Script.cpp

@@ -1207,17 +1207,7 @@ void scriptclass::run(void)
                     game.savedir = obj.entities[i].dir;
                 }
 
-                if (game.checkpoint_saving)
-                {
-                    bool success = game.savequick();
-                    game.gamesaved = success;
-                    game.gamesavefailed = !success;
-
-                    if (game.gamesavefailed) {
-                        game.show_save_fail();
-                        graphics.textboxapplyposition();
-                    }
-                }
+                game.checkpoint_save();
             }
             else if (words[0] == "gamestate")
             {