remove invalid HTML in Category List when Table headings 'of...

remove invalid HTML in Category List when Table headings 'off'

## Original Report

#### Steps to reproduce the issue
Menu item - Category List
Table headings switched off



#### Expected result

html:
tr class="cat-list-row0"
td class="list-title"
...

#### Actual result

html:
tr class="cat-list-row0" 
td headers="categorylist_header_title" class="list-title"

but no th tag in HTML output, so:
'A reference has been made to a non-existent ID. '

#### System information (as much as possible)

Joomla 3.3.6


#### Additional comments

HTML validation failed
Why not just replacing this with:

```php
// Put this somewhere above
$showHeadings = $this->params->get('show_headings', true);
?>
<thead <?php if (!$showHeadings): ?> class="hidden" <?php endif; ?>
```

This way headers would be still there but hidden which I also think is good for accessibility.