Simplify the way we annotate observable properties

We no longer add observable annotations to the constructor.
Instead, we just mark getters (and for read/write properties
also setters) with an observable annotation.

src/ol/iview.js

@@ -8,6 +8,7 @@ goog.require('ol.IView3D');
 /**
  * Interface for views.
  * @interface
+ * @extends {goog.events.Listenable}
  */
 ol.IView = function() {
 };