ol.ObjectAccessor#transform shouldn't dispatch an event to the target object

This PR fixes a bug in `ol.ObjectAccessor#transform`, where `transform` dispatches a `propertychange` event to the target object. There is no need to dispatch an event to the target object because setting `from` and `to` transformers has no impact on the target object. There is an impact on the source object though. So the event should be dispatched to the source object only.

Please review.