Save the id of [time_area]s into the savefile.

This should fix removing [time_area]s in a loaded game.

src/tod_manager.cpp

@@ -92,6 +92,7 @@ config tod_manager::to_config() const
 			t->write(area.add_child("time"));
 		}
 		area["current_time"] = i->currentTime;
+		area["id"] = i->id;
 	}
 	return cfg;
 }