Add Index for Dissemination field

[kaerumy]

For Dissemination boolean field, add an Index, so that we can do index searches via API or code.

See https://training.plone.org/5/mastering-plone/behaviors_1.html#add-a-index-for-the-new-field

Example for catalog.xml

<object name="portal_catalog">
<index name="featured" meta_type="BooleanIndex">
  <indexed_attr value="featured"/>
</index>
<column value="featured"/>
</object>