[FIX] web_editor: prevent dropping popup inside a substructure

Before this commit, it was possible to drag and drop a popup inside an
oe_structure which was itself inside an other oe_structure (e.g. snippet
parallax). It didn't make sense and moreover created bugs.

After this commit, it is no longer possible to drag and drop the popup
snippet or the newsletter popup snippet in a substructure.

task-2491976

closes odoo/odoo#81613

X-original-commit: e5041cc2
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Co-authored-by: qsm-odoo <qsm@odoo.com>