apiProposMixin: consider renaming property `fieldname` to `path`

[usu]

Since #2825: Besides normal field names, this property also accepts nested json paths (anything parseable by lodash get()/set() method).

Hence, path would be a better name instead of fieldname

[manuelmeister]

I'm not sure this is a good first issue?

[BacLuc]

Isn't this completed?

[manuelmeister]

Closed by #4746