Two concrete reasons to make the conversion: two issues are stymied by current tree.TreeNode.

1. #25090 tree.TreeNode requires an icon for every line.  Hence functions and classes get python file and director icons and they are difficult to remove.  By default, Treeview items do not get an image beside the text.

2. #20827 tree.TreeNode hardcodes single click behavior (in multiple places). TreeItems can only override or augment double clicks.  So it would be hard to keep hilites synchronized in browser and editor.  Treeview generates '<<TreeviewSelect>>' when a line is clicked on; setting the editor line can be done in a select handler.