[Unified Search] Filter editor clears value when changing target field

[davismcphee]

After adding a Unified Search filter, if I then try to change the target field name to another field, the value always gets cleared out. Presumably this is done because it's unknown if the operator or value applies to the new field, but it's probably fair to assume that if a user does this, their intent is to retain some part of the existing filter. It would be an improvement to the UX to retain the operator and value on field change whenever possible:

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)