The thinking here is that a user of a Tree may want to relate notes to other
parts of their UI, or other data in their application. While this could be
done by keeping a reference to the node itself, it could also be beneficial
to just track the ID. Given that ID is a public property of a TreeNode, but
given it doesn't currently have any other (public) purpose, this seems to
add some useful symmetry.
There's going to be a whole bunch of tests relating to the Tree and TreeNode
coming, let's make sure this ends up being fairly granular.
(side thought: it might be a good time soon to revisit all the tests for
Textual and try and wrangle them into some tidy structure)
