[FIX] web: BasicModel: recursively compute onchange specs

Before this rev., onchange specs where only computed up to level
2 (i.e. for fields inside x2many fields). So basically, if there
were a one2many list inside a one2many (e.g. which opens in a
form view), the fields of that inner one2many list weren't added
to the specs, and it could lead to unexpected responses from the
server (like erasing the changes made on the inner one2many
records).

Closes #19600