$marker element should be allowed everywhere

[oskarwrobel]

Model $marker is a temporary element created in the conversion process that helps setting markers from the input data to the Model#markers collection. Currently, this element is allowed only inside a $block and $root elements. It causes an issue with disappearing marker while setting data to the editor with a marker inside a table cell. Since $marker element is only a temporary element it should be allowed everywhere because a marker range does not have any limitations as long as is inside a $root bounds.