Simpler type for ol.events.Key

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()`.

src/ol/mapbrowserevent.js

@@ -154,7 +154,7 @@ ol.MapBrowserEventHandler = function(map) {
   this.dragListenerKeys_ = null;
 
   /**
-   * @type {ol.events.Key}
+   * @type {?ol.events.Key}
    * @private
    */
   this.pointerdownListenerKey_ = null;