#240 Duplicate ids possible in s5_html

[jamur2]

IDs in the s5_html templates may clash with node ids, generating duplicate ids in output. For example:

=====
Title
=====

Footer
======

text

Slide 2
=======

Generates:

...
<div id="footer">
...
<div class="slide" id="footer">
<h1>Footer</h1>
<p>text</p>
</div>
...

Which makes the "Footer" slide not display correctly.