I was pleased to meet Johannes Heinecke at the International Congress of Celtic Studies in Bangor last week. As well as producing a dependency treebank for Welsh, he has written a rather smart editor for CoNLL-U files, which are pretty much the standard these days for dependency trees.
I managed to get it working this morning on a Mac running Mac OS Mojave 10.14.6 with a minimum of hassle. You will need Java, Apache Maven, and Homebrew in order to install wget. One small surprise is that if you edit a file in a git repository then by default every time you edit the tree, the new file is committed, which makes the commit history look a bit busy.
The second best bit is that you can see non-projective relations at a glance, which I certainly can’t do in emacs.
The best bit, as someone who recently wrote a paper where all the arrows in the dependency diagrams pointed the wrong way and didn’t notice until the referees pointed it out, is that there is a wee button you can click on to get a tikz version of the tree for pasting into LaTeX.