Support non-fieldgroup steps

Problem/Motivation

Currently Multistep 6.x only supports fieldsets created by the CCK Fieldgroup module. Multistep does account for extra fields exposed through hook_content_extra_fields(), but only for placement within steps, not as potential steps themselves.

Proposed resolution

I admit to not finding the cck fieldgroup angle a very flexible solution as the only way to specify multistep steps. I would probably prefer a different UI for organizing these (maybe something closer to field_group in D7, but I'm thinking this would require a lot of factoring in D6).

Instead (for D6 only) how about a hook to alter the $groups variable in _multistep_block() before the menu items are generated? Something like:

       $groups = fieldgroup_groups($type);
+      // Support non-fieldgroup steps.
+      drupal_alter('multistep_fieldgroups', $groups, $type);

API changes

This way extra fields exposed through hook_content_extra_fields() could be added as steps. Something like:

function hook_multistep_fieldgroups_alter(&$fieldgroups, $type_name) {
  // Create an array of information about our pseudo field, as if it were a
  // fieldgroup element like Multistep expects.
  $pseudo_fieldgroup = array(
    'group_name' => $extra_field_name,
    'label' => $label,
    'settings' => array(
      'multistep' => array(
        'block' => TRUE,
        'step' => $step,
      ),
    ),
    'weight' => $weight,
  );

  // Add our pseudo field to the multistep fieldgroups array.
  $fieldgroups[$extra_field_name] = $pseudo_fieldgroup;
}

Modules that implement this hook would need to add their own re-sorting function for the array – or we could add that to Multistep after the hook… Either way, something like:

// Create a new dummy array for sorting, keyed by item weight.
  $dummy = array();
  foreach($fieldgroups as $item) {
    $dummy[$item['weight']] = $item;
  }
  // Sort by key.
  ksort($dummy);

  // Recreate fieldgroups from the dummy array order, now sorted by item weight.
  $fieldgroups = array();
  foreach($dummy as $item) {
    $fieldgroups[$item['group_name']] = $item;
  }

Or we could skip the hook and add support for extra fields more generically… I just wrote some code that would work for that, but we'd need a UI to declare extra fields as steps. I'll start by adding a patch for the hook for starters… it would be good to get some feedback on whether others see this as useful before taking this too far in D6.