More XML syntax fixes

Another batch of XML syntax fixes.  @LaurentGoderre I noticed a few things while making these fixes:

### Lightbox AJAX demo

The following 2 pages are only meant as HTML snippets for the lightbox ajax demo.  I had to add html and body tags to get the parsing to continue:

* lightbox/ajax/ajax-eng.html
* lightbox/ajax/ajax-fra.html

### Ending markup not stripped

Some demo.src files didn't have the markup after `<!-- MainContentEnd -->` stripped:

* slider/*.html
* theme-base/*.html
* theme-clf2-nsi2/*.html
* theme-wet-boew/*.html

### No markup stripped

Some demo.src files didn't have any markup stripped (they just looked like copies from the wet-boew/demo folder).  Seemed to be when the filename doesn't have a `-eng` or `-fra` suffix:

* theme-base/ar/*.html
* theme-base/es/*.html
* theme-gcwu-fegc/ar/*.html
* theme-gcwu-fegc/es/*.html
* theme-wet-boew/ar/*.html
* theme-wet-boew/es/*.html
* tableparser/*.html