[FIX] SSF: modifier based on m2m value

m2ms are internally represented as "6" commands, however in domains
it's possible to compare an m2m value to a list of ids (to
investigate: whether this is an artifact of internal webclient repr or
part of the real contract).

Add a workaround in SSF modifier computation to convert the m2m
command storage to a simple ids list.

A better fix would probably have been to represent the m2m as a list
of ids internally (and only convert on load / save) however it not
completely trivial as it has to be done recursively in order to
properly handle an m2m inside an o2m. So it's a complete change of the
internal data model (which should probably go alongide more
fundamental changes e.g. properly handling parent refs, etc...)

Also add very minor support for widgets (mostly so it's possible to
set widget=many2many on an o2m field).

closes odoo/odoo#39433

Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>