Replace DefaultTableMapping::setFieldNames() with ::add*() and ::remove*()

Opened a merge request with the change. This will need to be changed to deprecate setFieldNames() before it can go in, but I would like some discussion/agreement that this makes sense first, before going ahead with that. Thus, marking "Needs review".

Maybe it needs to deprecate the old confusing methods?

This looks sensible to me but two things:

1. Agreed with #5 we should deprecate the old method(s) that are being replaced here.

2. There are @todos to make the new methods protected, but they should just be added as deprecated from the start if that's the intention.

Right, as noted in #3 the plan is to deprecate the existing method.

Re marking the methods as protected: I actually disagree with the notion of that @todo, i.e. with what is being pursued in 📌 Convert DefaultTableMapping::setFieldNames() and ::setExtraColumns() to protected methods Active . In my opinion we should just remove the @todo and leave the method(/s) public. But since the existing method has that todo it seemed sneaky to just remove it wholesale, just because I disagree with it. On the other hand, it also doesn't seem fair to introduce it as protected right from the get-go if there is no consensus on 📌 Convert DefaultTableMapping::setFieldNames() and ::setExtraColumns() to protected methods Active yet.