Rewrite addon/plugin handling

Intensive tests required. Webservice must allow for use of addons as well. Also: Improve the way addons are used by ServerPackCreator, and, subsequently, improve the exampleaddon. Best way would be something like calling the main from any addon inside the addonfolder with the current configurationModel, instead of executing the JAR in a process, with the configurationModel passed as a commandline-argument... This looks promising: https://github.com/pf4j/pf4j