ox-gpgweb: Implement virtual headlines and HTML_CLASS property.

This also chnanges how HTML_CONTAINER_CLASS is used. That is not a
problem here becuase we are cot yet using that property. With these
changes the following markup is possible (example):

• Top header :PROPERTIES: :HTML_CLASS: centered :END:
  • <div class="pure-g"> :html:virtual:
  • Foo :PROPERTIES: :HTML_CONTAINER_CLASS: pure-u-1 pure-u-lg-1-3 :END:
    
    Foo is about bar.
  • </div><!-- end pure-g --> :html:virtual:

The HTML_CLASS property puts a class="centered" into the <h> tag.

The HTML_CONTAINER_CLASS property encloses its <h> section with
another div:

<div class="pure-u-1 pure-u-lg-1-3">
<h3>Foo</h3>
<div class="outline-text-1" id="text-1-2">
<p>Foo is about bar.</p>
</div>
</div>

The "html" tag (the "virtual" is right now only informational) inserts
the given HTML directly at this place into the output. Note that this
can't be done with #+HTML: because that would distrub the sectioning.