Store the feature's commonly used id

To not clobber the feature's attributes, this is a separate
member property.

src/ol/feature.js

@@ -14,6 +14,12 @@ ol.Feature = function(opt_values) {
 
   goog.base(this, opt_values);
 
+  /**
+   * @type {string|undefined}
+   * @private
+   */
+  this.featureId_;
+
   /**
    * @type {string|undefined}
    * @private
@@ -46,6 +52,17 @@ ol.Feature.prototype.getAttributes = function() {
 };
 
 
+/**
+ * Returns the feature's commonly used identifier. This identifier is usually
+ * the unique id in the source store.
+ *
+ * @return {string|undefined} The feature's identifier.
+ */
+ol.Feature.prototype.getFeatureId = function() {
+  return this.featureId_;
+};
+
+
 /**
  * @return {ol.geom.Geometry} The geometry (or null if none).
  */
@@ -85,6 +102,17 @@ ol.Feature.prototype.set = function(key, value) {
 };
 
 
+/**
+ * Set the feature's commonly used identifier. This identifier is usually the
+ * unique id in the source store.
+ *
+ * @param {string} featureId The feature's identifier.
+ */
+ol.Feature.prototype.setFeatureId = function(featureId) {
+  this.featureId_ = featureId;
+};
+
+
 /**
  * @param {ol.geom.Geometry} geometry The geometry.
  */