Add an extension point at the end of ComponentManager initialization

The goal is to make possible to anyone to add component to ComponentManager dynamically and not loose any startup event/information or behind lost by components like EventManager which statically get the list of listeners at init (someting that needs to be fixed of course but some other component could have the same issue).

The target is extension manager initialization: see http://dev.xwiki.org/xwiki/bin/view/Design/ExtensionManager