Convert tabs rendering to forms api/drupal_render

Tabs are currently generated only partially through the forms api. It would be better to generate them fully through the forms api, so that developers could use tabs seamlessly in forms.

Attached patch does this. It will necessitate changes in how tabs are generated. Old way:

  $tabs = array(
    '#name' => 'example1',
  );
  $tabs[] = array(
    '#title' => t('One'),
    '#content' => t('First tab content.'),
  );
  $tabs[] = array(
    '#title' => t('Two'),
    '#content' => t('Second tab content.'),
  );
  $tabs[] = array(
    '#title' => t('Three'),
    '#content' => t('Third tab content.'),
  );
  
  return theme('tabs_tabset', $tabs);

New way:

  $form = array();

  $form['example1'] = array(
    '#type' => 'tabset',
  );
  $form['example1']['tab1'] = array(
    '#type' => 'tabpage',
    '#title' => t('One'),
    '#content' => t('First tab content.'),
  );
  $form['example1']['tab2'] = array(
    '#type' => 'tabpage',
    '#title' => t('Two'),
    '#content' => t('Second tab content.'),
  );
  $form['example1']['tab3'] = array(
    '#type' => 'tabpage',
    '#title' => t('Three'),
    '#content' => t('Third tab content.'),
  );

The patch also includes an example module implementing tabs.