fix: TOC element with 'hidden' attribute should be hidden in the page but visible in TOC panel
fix: TOC element with 'hidden' attribute should be hidden in the page but visible in TOC panel

TOC element with 'hidden' attribute, e.g., `<nav role="doc-toc" hidden>` is used
when the TOC should be hidden in the printed page but should work in the TOC panel
in the Viewer UI.

vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js