[FIX] web: handle widget order and custom sequence
[FIX] web: handle widget order and custom sequence

An editable list can be drag-and-drop sorted by:

- displaying a sequence field,
- displaying a field with the `handle` widget

At first it was only working if the list was not specifically ordered,
but with 26bf35260 it also worked if the list was ordered on `sequence`
or `sequence ASC`.

This commit changes this so it works:

- if the list is not ordered (same as the original intention),
- if the list is ordered by the sequence field (widget `handle` or sequence)

This allows to have a custom model (on which we can't define _order) to be
ordered on a custom field.

This also remove a far-fetched oddity such as having the view ordered by
sequence but having a custom sequence field before `sequence`.

opw-704612

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,dfang/odoo,dfang/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,dfang/odoo,dfang/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo