[idea]: item group enhancement

  1. show only is_group = 0 item group in item group dropdown list

  2. add description(template) in item group, set item_group.description in for description field in item
    this can server as a simplified solution for item variants with multi attributes, because the current item variant does not support manual attribute value entry even if rename attribute opton activated, the auto generated item name does not take into consideration the manually entered attribute value(text)

  3. when search high level parent node, all items assigned to its children item groups should be listed.