Fix selection position in a post-fixer

[pjasiun]

Since now it should be possible to write also postfixes for selection, we should try to replace Schema.getNearestSelectionRange (moved from document) with a postfixer which will automatically move the selection to the proper place.

[pjasiun]

See also: https://github.com/ckeditor/ckeditor5-engine/issues/1228.

[Reinmar]

I changed the title of this ticket because it's not about replacing the existing method with something, but about creating a post-fixer. What happens with this method is a side topic.

[Reinmar]

A use case to consider: https://stackoverflow.com/questions/49358150/is-there-any-way-to-select-all-the-content-of-ckeditor-5-by-code/49358509#49358509

[Reinmar]

I think it's done. @jodator's selection postfixer was needed for tables but it covered far more cases and fixed many issues.