Tree example needed

[mark-summerfield]

I would like to use the Tree widget but the docs aren't sufficient.

I need to know how to:

• Clear all the items from a Tree
• Programatically select or deselect an item in the Tree
• Access all the selected items in the Tree (e.g., so I can read their texts)

I want to see if I can use Trees rather than MultiBrowsers for the Sections and Tags in DebFind; see debsearch/cmd/gui/app.go.