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/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) {