Instead of having `ol.events.Key` be a listener object or an array of listener objects, it should be less error prone to have it just be a single listener object.
To avoid using too many functions with multiple return types, the `ol.events.*` functions for registering and unregistering listeners no longer accept an array of event types (and only a single key is returned when registering).
To make it convenient for users to register multiple listeners at once, the `observable.on()` method accepts an array of event types. Internally in the library, we should use the less risky `ol.events.listen()`.
