Allow altering of style property groups

the attached patch adds a drupal_alter to the function wysiwyg_filter_get_style_property_groups.

Example usage:

/**
 * Implementation of hook_wysiwyg_filter_style_property_groups_alter().
 */
function mymodule_wysiwyg_filter_style_property_groups_alter(&$groups) {
  $groups += array(
    'page-break' => array(
      'title' => t('Page break'),
      'properties' => array(
        'page-break-after' => '(?:auto|always|avoid|left|right|inherit)',
        'page-break-before' => '(?:auto|always|avoid|left|right|inherit)',
        'page-break-inside' => '(?:auto|avoid|inherit)',
      ),
    ),
  );
}