Support toc-title in revealjs (#7171)

* Support toc-title in revealjs
* Add semantic HTML "nav" tag

Closes #7170.

As with default.html5

data/templates/default.revealjs

@@ -59,7 +59,12 @@ $endif$
 $endif$
 $if(toc)$
 <section id="$idprefix$TOC">
+<nav role="doc-toc"> 
+$if(toc-title)$
+<h2 id="$idprefix$toc-title">$toc-title$</h2>
+$endif$
 $table-of-contents$
+</nav>
 </section>
 $endif$