Rename ol.Collection#getAt to ol.Collection#item

TouchList, DOMStringList, DOMTokenList, NodeList all have an
`item(index)` function to retrieve an item from a list.
We should do the same.

src/ol/collection.js

@@ -144,9 +144,9 @@ ol.Collection.prototype.getArray = function() {
  * Get the element at the provided index.
  * @param {number} index Index.
  * @return {*} Element.
- * @todo api
+ * @todo api stable
  */
-ol.Collection.prototype.getAt = function(index) {
+ol.Collection.prototype.item = function(index) {
   return this.array_[index];
 };
 

src/ol/interaction/selectinteraction.js

@@ -135,7 +135,7 @@ ol.interaction.Select.prototype.handleMapBrowserEvent =
         }, undefined, this.layerFilter_);
     if (goog.isDef(feature) &&
         features.getLength() == 1 &&
-        features.getAt(0) == feature) {
+        features.item(0) == feature) {
       // No change
     } else {
       if (features.getLength() !== 0) {