'Searched fields' option to search all fields if none selected should not save them

Problem/Motivation

The Searched Fields form element says:

> Select the fields that will be searched. If no fields are selected, all available fulltext fields will be searched.

That should mean that if I save a Search Page, and then add more fulltext fields to my index, they should be included.

However, that is not the case.

Saving the form sets all the *current* options into the config, and so if the form is reloaded after a new field has been added to the search index, it's not selected.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes