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/layer/layergroup.js

@@ -28,8 +28,6 @@ ol.layer.GroupProperty = {
  * @constructor
  * @extends {ol.layer.Base}
  * @param {olx.layer.GroupOptions=} opt_options Layer options.
- * @todo observable layers {ol.Collection} collection of {@link ol.layer} layers
- *       that are part of this group
  * @todo api
  */
 ol.layer.Group = function(opt_options) {
@@ -142,7 +140,9 @@ ol.layer.Group.prototype.handleLayersRemove_ = function(collectionEvent) {
 
 
 /**
- * @return {ol.Collection|undefined} Collection of layers.
+ * @return {ol.Collection|undefined} Collection of {@link ol.layer.Layer layers}
+ *     that are part of this group.
+ * @todo observable
  */
 ol.layer.Group.prototype.getLayers = function() {
   return /** @type {ol.Collection|undefined} */ (this.get(
@@ -155,7 +155,9 @@ goog.exportProperty(
 
 
 /**
- * @param {ol.Collection|undefined} layers Collection of layers.
+ * @param {ol.Collection|undefined} layers Collection of
+ * {@link ol.layer.Layer layers} that are part of this group.
+ * @todo observable
  */
 ol.layer.Group.prototype.setLayers = function(layers) {
   this.set(ol.layer.GroupProperty.LAYERS, layers);