Rename ol.Observable#dispatchChangeEvent() to #changed()

Since dispatchChangeEvent does not only dispatch a change event, but
also increases the revision counter, the method name should reflect
this.

src/ol/source/imagevectorsource.js

@@ -204,7 +204,7 @@ ol.source.ImageVector.prototype.getTransform_ =
  */
 ol.source.ImageVector.prototype.handleImageChange_ =
     function(event) {
-  this.dispatchChangeEvent();
+  this.changed();
 };