[FIX] models: in method onchange(), check for record dirtiness only on *2many fiels

Cascading onchanges can be caused by a related field computed in cache.  This
causes a bug in sale order lines, were setting the uom field forces reading
product fields, which are inherited from product templates.  The inherited
fields are computed as related fields, which marks the product record as dirty.
This subsequently triggers an onchange on the product field, which resets the
uom field!