Adds some changes to List and Tree API.

Generally mostly code comments and API renaming.

• List Data is now called Items
• Tree Data is now called Children
• Instead of Enumerator returning two values for (indent string, prefix string). We now split this into a separate Indenter which handles indent and Enumerator which handles prefixes.
• Renamed StringNode to Leaf (which is a TreeNode without children, what the StringNode was representing.).