Scala 3 maintenance

[OndrejSpanel]

I have some difficulties with the Tree wrapper in my Scala 3 project - the path dependent types like companion.CellInfo make it hard to create derived classes for Tree or Editor.

At this point only SBT, compiler and libraries are updated to recent versions, which is probably not worth a release.

[OndrejSpanel]

To my surprise the version here already solved all Scala 3 problems I have. This was done in #22, but I have a local copy of a Tree class and a few related classes in my project which I did not update. Once I have updated it all errors are gone.

My forked version contains a few improvements and fixes, like Link support in drag and drop and support for null values in tree events.

[OndrejSpanel]

I have ported a few simple fixes from my local copy. The rest is more difficult, it ads copy / paste support in addition to the drop support. Unless someone is specifically interested in that, I think it is not worth the effort.

[benhutchison]

@OndrejSpanel in truth I make no use of Swing now, and this library is probably just for existing users like yourself. Happy to merge and publish binaries when requested.