Better support for search-index optimization in generated HTML help

[modelica@…]

In order to better support the search indexing of the generated Modelica HTML files, it would be very useful that the autogenerated table of the package contents has a separate h2 class, such that it can be distinguised from the the desired description or revisions. Here's some example what I mean

<h2><a id="info" name="info">Information</a></h2>
<p>This is the desired description ... </p>

<h2>Contents</h2>
<table><!-- it follows the autogenerated table of contents --></table>

<h2><a id="revisions" name="revisions">Revisions</a></h2>

All three h2 headings cannot be distinguished.