ComponentDescriptorAddedEvent events are not sent for core components

This is because, in XWikiServletContextListener, we call

ecm.initialize(this.getClass().getClassLoader());

which registers all the components found in components.txt but at this time, there's no ComponentEventManager set in the Component Manager and thus no event is sent for them.

The solution should be to set the StackingComponentEventManager in the ComponentManager before ecm.initialize() is called.