Add `index` method to Component API

src/dom_components/model/Component.js

@@ -188,6 +188,15 @@ const Component = Backbone.Model.extend(Styleable).extend(
       return !!(this.get('type') == type);
     },
 
+    /**
+     * Get the index of the component in the parent collection.
+     * @return {Number}
+     */
+    index() {
+      const { collection } = this;
+      return collection && collection.indexOf(this);
+    },
+
     /**
      * Find inner components by query string.
      * **ATTENTION**: this method works only with already rendered component