Conversion utilities: Unify parameters received by callbacks in downcast and upcast configuration

[scofalik]

Follow-up to ckeditor/ckeditor5-engine#1274.

Right now, in element-to-element conversion, if we pass a callback creating an element, the callback is called with different parameters for upcasting and downcasting.

For downcasting it is ( modelItem, consumable, conversionApi ).

For upcasting it is ( viewItem, modelWriter ).

We should unify them. Callbacks for downcasting are fired like this because of the good-ol' converters we already had. We need to think whether we really need consumable and whole conversionApi in those callbacks. Maybe we will settle on: ( item, writer, conversionApi )?

[pjasiun]

No only callbacks (creators), but also converters API should be unified. Can fix also https://github.com/ckeditor/ckeditor5-engine/issues/1261.