[FIX] web_editor: can set bg position on root editor

Scenario:

- go to /slides and start editing the page
- change the position of background banner
- save

=> traceback

Why:

The code for pan tool duplicates the target element in an overlay. In
the given use case, it means a node with .o_editable class is created
that will cause an error when saving because the code expect the cloned
element to be an editor (in `RTEWidget.save()`) but it is not (and is
eg. missing `.data('options')`).

opw-2427560

closes odoo/odoo#66639

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>