Update Commentary header.

Update Commentary header.
(tree-widget-theme): Doc fix.
(tree-widget-space-width): New option.
(tree-widget-image-properties): Look up in the default theme too.
(tree-widget--cursors): Only for images with arrow pointer shape.
(tree-widget-lookup-image): Pointer shape is hand by default.
(tree-widget-icon): Generic icon widget renamed from
`tree-widget-control'.
(tree-widget-*-icon): Rename from `tree-widget-*-control' and
derive from `tree-widget-icon'.
(tree-widget-handle): Improve default look and feel of the text
representation.
(tree-widget): Rename :*-control properties to :*-icon properties.
Add :action and :help-echo properties.
(tree-widget-after-toggle-functions): Move.
(tree-widget-close-node, tree-widget-open-node): Remove.
(tree-widget-before-create-icon-functions): New hook.
(tree-widget-value-create): Update to allow customization of icons
and nodes at run-time via that new hook.
(tree-widget-icon-create, tree-widget-leaf-node-icon-p)
(tree-widget-icon-action, tree-widget-icon-help-echo)
(tree-widget-action, tree-widget-help-echo): New functions.