Allow headers to make use of the hdicon

This allows the category_headers to make use of the header icons we added.  There were 3 cases that I found so here there are, plus an example done to managefeatures.controller since it used all three cases.  

I think the extension of the icon is useful even if we don't use,  

Please **provide your feedback** on if it adds or distracts/clutters or perhaps just use the generic 'cog' config icon in most places or leave things as they are (void) but with the capability added to the functions.

How to use:  
- If the title is set with a  ```$context['settings_title'] = ''``` then you optionally add in a  ```$context['settings_icon'] = ''```  this is used by the admin template
- If the title is set in the $config_vars array like  ```array('title', '')``` then you change that to ```array('title', '', 'icon' => '')``` this is used by the settings class
- If the title is set in the $listOptions  array like ```'title' => $txt['']``` then you add in a new option as ``` 'icon' => ''``` this is used by createList