[FIX] web, website: fix menu auto hide in edit mode

Steps to reproduce:

- Go to Website > Add menu items in a way that activates “auto-hide” (to
set the overflowing menu items in a “+” dropdown) if the viewport was
resized.
- Go to “edit” mode (adding the sidebar reduces the current window
width) > The “auto-hide” menu adaptation is disabled, and overflowing
menu items are still visible.

The goal of this commit is to fix the behaviour described above (and
potentially, issues that can result from the editor's "unbreakable"
rollbacks on top menu) by preventing the unbreakable mechanism from
detecting header changes and cancelling the auto-hide updates.

Related to opw-3484742

closes odoo/odoo#135904

X-original-commit: 4f6629d3
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>