example: fix simplecms layout loading
Fix broken tabs/orphaned content at add form error pages.

When the add page has form errors, the placeholders will still have ""
as id. Some placeholders may already be marked by the DELETE field too.

Changes:
- read the `placeholder_slot` field in `dom_placeholders` too to find
  the correct placeholder.
- include the DELETE field in the HTML when the server re-renders the page.
- pass the 'delete' flag to the cp_data.set_placeholders() call too.