Remove third-party code from the source tree

[Kernald]

There's currently a lot of code in the repository that isn't from PlantUML directly. It makes it quite tricky to determine if something might be fixed directly in-tree, where to report the issue if not, knowing which version is currently in use... This wiki page lists a bunch of the dependencies and their respective Maven coordinates. It would be great to remove them from the source tree and depend on them in a proper way.