TOC: Add collapse/expand options

Large specifications with many (nested) sections are painful to navigate
through when the TOC is expanded by default. Introduce four new options,
"Expand/Collapse whole section" is based on Kate's document view while
"Expand/Collapse all" was added to handle the top-level sections.

As for other viewers, PDF.js uses shift-click to handle the former while
using double-click on a the TOC icon to handle the latter. That is not
very obvious, so extending the context menu seems the next best option.

BUG: 216870

Differential Revision: https://phabricator.kde.org/D14904