Allow programmatic read access to the configuration

Some web libraries are interested in the web app configs and parse web.xml/web-fragment.xml themselves. Examples.

* Mojarra's com.sun.faces.config.ConfigureListener.WebXmlProcessor: checks for the FacesServlet, error-page and 'distributable' definitions.
* OmniFaces' org.omnifaces.config.WebXml: welcome files, error pages, form login page, form error page, security constraints, session timeout.
* PrimeFaces' org.primefaces.config.ConfigContainer: error pages.

There are proposed additional programmatic configuration APIs: #48, #50, #70. Their implementation would mean that those interested web libraries won't be able to see the programmatically configured things. Thus the need for a corresponding configuration reading API.