Remove restriction on filter fields only being fields displayed

[richsage]

If I display the following fields in my List eg:

firstName
lastName
country

by using:

$this->addFields(array(
        'firstName,
        'lastName',
        'country',
))

and I want to filter (advanced) on another, non-displayed field, I'm not able to since the getAdvancedFilters method in DataManager\Base\Action\ListAction.php uses the getFields() method to iterate over. Since this method is private, I can't override it to provide my own implementation.

Solution: Either allow this to be configurable via a closure, or switch private for protected.