[FIX] web_editor: fix oe_structure view that are not in extention mode

Issue occurs due to the way we compute the default value in base ir ui view.
The method _compute_defaults don't set extention mode under some condition
even if we provide an inherit_id view.

This commit double fix it, we use now self.env['ir.ui.view'] to create the
new view and so always compute the mode based on 'if inherit id or not'.
Second fixes is to explicitely set mode manually as 'extention'.

before commit:
when you drop snippet to blog sidebar and click save button.
The changes of user is not visible in sidebar because view created in mode
'normal' and not 'extention'

task-2311520
closes odoo/odoo#55908

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>
Co-authored-by: jpr-odoo <jpr@openerp.com>