Generate TypeSystemHolder as .java (in sources) instead of .class (in resources)

Up to XmlBeans 5.0.0, the TypeSystemHolder class is generated as .class in the resources.

I guess the reason was on-the-fly compilation and preloading the resources.

This usecase seems to be out-of-date and the generated .class file causes some inconveniences with current build tools and IDEs. Therefore I've decided to remove the byte-logic of tweaking an existing TypeSystemHolder template and generate the Holder from scratch into the sources.