type=standard_combobox and type=editable_combobox layout elements

<layout type="standard_combobox" id="option-list">

  <!-- ... -->

  <append_items>
    <items>
      <label>Option 1</label>
      <label>Option 2</label>
    </items>
  </append_items>

</layout>

type=standard_combobox and type=editable_combobox layout share the same elements as type=list layouts. The combo-box layout managers inherit from the list layout manager, and inherit the list layout manager's methods.