Even if the model/default bylaw says the unit owner cleans the window, an OC can change its bylaws. So, it seems a sensible thing to put this beyond doubt would be resolve at a general meeting to amend the bylaws. A new bylaw could state that the OC shall engage a professional cleaner to clean the exterior surface of the building as necessary, including cleaning of the windows. If everyone, or nearly everyone, finds it difficult to clean their windows then this should get the necessary special resolution support.