Backport from 10: No spacing between paragraphs in CKEditor edit mode

When filling the content with the ckeditor, there is no visual way to distinguish between a paragraph <p> which is set when the editor presses [Enter] from a line break <br> when he uses [Shift]+[Enter].

The former xinha editor was using different interline spaces for the two cases. It would be nice to change the default ckeditor style in the same way.