theme_table() takes an optional footer variable and produces <tfoot>

This was a backport of https://www.drupal.org/node/2282663

In D7 the theme_table() function takes an optional footer variable:

 *   - footer: An array of table rows which will be printed within a <tfoot>    
 *     tag, in the same format as the rows element (see above).                 
 *     The structure is the same the one defined for the "rows" key except      
 *     that the no_striping boolean has no effect, there is no rows striping    
 *     for the table footer.