Move libtiled-java to its own repository #3277

It would make sense for the libtiled-java library to get its own repository under the mapeditor organization. There it can have its own workflows and its own maintainers.

I'm not personally using libtiled-java and I'm also not much of a Java developer anymore, so it would be good if we could find some new or additional maintainers for this library.

The split should be done in a way that keeps the development history intact, so likely using git filter-branch.