Aligned to the changes in engine

[pomek]

Suggested merge commit message (convention)

Internal: Aligned to the changes in the engine. See ckeditor/ckeditor5#742.

Requires: ckeditor/ckeditor5-engine#1389.

[coveralls]

Coverage remained the same at 100.0% when pulling 6fe73b1 on t/ckeditor5/742 into 1069d44 on master.

[Reinmar]

For the future reference, cause I'm pretty sure I'll be wondering why we haven't changed the option.class to option.classes in this case.

The thing is that the option describes the class of the marker (like in the "class of problems"). It's the type of that marker. Not a generic property of some view element's definition.