Add filter attribute to relation fields
In some cases it would be very useful if Many2One and Many2Many fields could have a "filter" attribute with a domain expression.
The incremental search should honour that filter and it would be used as default filter in the search dialog just like search_value field in ir.action.act_window. But the user could remove the filter and choose any record the domain allows.
Of course, the filter expression would not be used as a constraint on the server.
The use case for this is that in some cases it is desirable to help the user on the search but there's no strong reason to enforcing a domain.