[FIX] fields, web: improve form onchange to x2many

In a form view, when a field onchange lead to a change on a x2many,
there was two different behavior:

- if the x2many had an embedded view (eg. a tree view inside a form
view) the onchange would notify that it expected the x2many field in
this embedded view to be changed and handled the changes correctly.

- if the x2many had a default view, the onchange ORM would not be
aware the x2many could be modified and would not sent the changes
back causing blank or not updated x2m lines and error on save.

---

Two solutions were birthed to solve the second point:

=> PR #10557 = solving everything

With this PR the onchange in the ORM is aware of every fields in the
current view (even field in a x2m in a x2m in a x2m in a form view) and
if any of these are change the javascript gets back the value of the
fields present in the view.

This PR has currently not been merged by fear of changing too much and
anyway could only be done in master.

=> PR #12249 = if no field for x2many, send its form view fields

With this change, if the ORM onchange is not aware of the fields in the
x2many widget to returns, all the field in the x2m default form view are
returned.

This was merged in bbdf9605 but introduced a number of other issue:

- in most situation the x2many is represented by a list view, which may
have fields missing of the form view,  so the original is still present.

- the view used may differ from the default form view in other way
(depending on value in context or other possibilities).

- the form view could have fields not present in the form view which
could end up in `write` on fields which should not be written to.

---

This commit reverts bbdf9605 and adapts a small part of #10557 so the
x2many with default view works as an embedded x2many. For more than one
level (eg. a x2many in a x2many) this would still not work but it is
only solvable by a PR such as #10557 which could only be targetted for
master.

With this commit:

- the list of fields sent to ORM onchange is computed at the first onchange
- the fields from a x2many field default view is sent for onchange
- the initial onchange on record creation is delayed to when x2many are loaded

closes #12249, closes #15336, closes #15890
fixes #11236, fixes #12249, fixes #15129, #15419
opw-705965 opw-716095 opw-715619 opw-710440

note: already present in 10.0 (with a test) as f65475d6