Defining length mapping.

[pjasiun]

Now length of the view element in the model is always 1 (1 view element responds 1 model element) and the length of the text is the text length (view text length responds model text length). It should be possible do define length mapping so, for example:

• one model element could be mapped to 2 view elements,
• some view text could have model length 0, so they have no corresponding model text.

Because of the performance of the mapper, instead of using events, there should be a setLength function which store the lengths in the map where the access is quick.

[scofalik]

This has been resolved in ckeditor/ckeditor5-engine#584 by introducing Mapper#registerViewToModelLength function that takes a view element name and a function that evaluates view element length in the model.