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/deviceorientation.js

@@ -132,7 +132,7 @@ ol.DeviceOrientation.prototype.orientationChange_ = function(browserEvent) {
     this.set(ol.DeviceOrientationProperty.GAMMA,
         goog.math.toRadians(event.gamma));
   }
-  this.dispatchChangeEvent();
+  this.changed();
 };