Change-based conversion can be really confusing

[Reinmar]

If things like https://github.com/ckeditor/ckeditor5-image/issues/31 are required, then we have a problem, either with the concept itself or how we sell it.

As we know, the concept of converting a change has its merits and it's unlikely that we'll be able to change it. It allows for a better composition of features and reduces overhead on the changes coming from the collaboration. If we'd like to have dirty fragments conversion, we'd need to translate applied changes to regions to re-convert, including things like selection and markers. Taking feature composition into consideration, it is very hard to imagine such a solution and, very likely, it wouldn't be a clean and clear concept either.

Anyway, apparently we need to work on how we sell the conversion. E.g. if each model change requires its own converter, then converter builders should not propose consuming more than either an element or an attribute. Which means that perhaps:

• We could have some warning that something wasn't consumed (IIRC, this was requested a couple of times already :D). In this specific case, AFAIU, the image src attr wasn't consumed because the builder didn't do that.
• There should be no generic converterBuilder.from() method (in fact, I see that it's been just removed, now, only the specialised ones are public).

And finally, we need to be very clear about this in the documentation, other places in the API (like – if you have a converter function, the name should always state whether it converts an element or attribute), etc.

[Reinmar]

Part of https://github.com/ckeditor/ckeditor5-engine/issues/829.