This encourages importing it from `textual.widgets.tree` instead, keeping it
in line with the other changes made for #1637.
Note this is a breaking change.
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.
This is sort of different from selected. Selected is when someone mashes the
enter button or clicks on a node. Highlighted is when the cursor moves into
a new node.
See #1400.
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)
