support a form of modular XML configuration file

I realise the history of wildfly & that things used to use many XML files and there was a move to unify things. But some modularity of configuration can be a good thing

With layered products it would be handy to allow separate XML configuration files to configure separate layers/features/parts. e.g. one for web, one for FSW, one for Infinispan/JDG, one for Portal and so forth. e.g. a layered product could release its own XML configuration file by default.

Rather than it being subsystem specific (e.g. each layer may need to define a database) its more a way of grouping parts of the XML into a separate underlying source XML file.

e.g. so folks could combine any of the (say) 4 XML files for web / FSW / JDG / Portal to make a configuration for a new wildfly container with different pieces depending on how folks want to map features to JVMs.

e.g. rather than specifying a single XML file to startup, we'd allow specifying N different XML files with 1 configuration file being the default file used for adding new subsystems; otherwise the configuration system would remember which file a particular bit of configuration comes from; so if its changed, it knows which configuration needs to be written to which file.