Java - Add option to configure constants filename

Issue 1090 raised the question of the name of the file generated for java constants.

Decision was taken to change from "Constants.java" to "java_package_nameConstants.java"

It was suggested that the change in the name generation could be made optional, and that's exactly what I need today. Since I'm working on quite old projects, it's a huge pain to change all code to use new filenames.

As it was said, it should represent a very small amount of time to make this an option.

Edit : added a patch proposition

Thanks