[FIX] models: reflect inherited (by `_inherits`) custom fields

When a custom field is created, it now appears in `ir.model.fields` on the
model's children models (by `_inherits`).  This makes the reflection more
accurate (inherited custom fields are visible), although not perfect (they
appear as "base" fields and thus cannot be overridden).

The reflection of fields in `ir.model.fields` has been improved to deal with
custom field creation, modification and suppression.

Note that custom fields are still not inherited through `_inherit` relations.

opw-687626
opw-645065