theme_table and theme_links accessibility

This is a small patch to make the already very powerful theme system even more accessible and functional.

theme_table: This patch will allow you to tag cells as headers to make the already available functionally of _theme_table_cell accessible to you. By adding a header key with either one of the values 1, true, or `th` theme_table can now generate headings for you on the side of the table in a row in the centre anywhere you want them.

This is already implemented but duplicated in the latest patch for the modules administration page which makes the modules screen look so much neater with all the packages in one table and all the rows lined up. Each package heading can be table headings and even without style sheets are already styled appropriately.

ex:

    $cells[] = array('data' => 'Cell 1', 'header' => 1);
    $cells[] = array('data' => 'Cell 2', 'header' => true);
    $cells[] = array('data' => 'Cell 3', 'header' => 'th');
    
    $rows[] = array(
        'data' =>    $cells,
        'style' => 'font-weight : bold;'
    );
    return theme('table',array(),$rows);    

theme_links: As a newcomer I found using this function very frustrating due to its complexity and lack of documentation which is a pity because this is a very powerful piece of code. Not only will it generate you a list of links but it will also surround some text with span tags and you can add custom attributes to these albeit by using three nested arrays. To simplify this process and be more consistent with other elements for example row, header or cell you can now also add attributes right next to title and href or use the old way of combining them in an attribute array.

ex:

     $output = theme('links', array(array(
                'title' => t('span'),
                'href' => 'testthemepatch/test',
                'class' => 'modules-heading',
                'style' => 'font-weight : bold;'
                )
        ));
    return $output;

In stead of:

     $output = theme('links', array(array(
                'title' => t('span'),
                'href' => 'testthemepatch/test',
                'attributes' => array(
                        'class' => 'modules-heading',
                        'style' => 'font-weight : bold;')
                )
        ));
    return $output;

Your comments and suggestions are welcomed and appreciated.