Description of the extender pattern configuration. The extender tracks extensions. The particularity of this architecture-style is that extensions are packaged in different bundles. An extension is detected by analyzing the bundle. The mark is currently a header in the bundle manifest. At each time a matching bundle appears or disappears, a callback is invoked. Declaring the method to invoke when a matching bundle arrives Declaring the method to invoke when a matching bundle leaves Declaring the looked manifest header.