Make the Unicode tree widget prettier

Make the Unicode tree widget prettier

* lisp/tree-widget.el (tree-widget-nohandle-guide): New widget:
"One small change is needed to accommodate this
style. `tree-widget-guide’ specifies a single vertical bar “guide”
widget. In the unicode tree, two separate vertical bars are needed
— one “plain” (│), and one which connects to the horizontal
“handle” preceding the node (├)."
(tree-widget--cursors):
(tree-widget):
(tree-widget-value-create): Use it.

Copyright-paperwork-exempt: yes