Add possibility to scroll down / up several lines via page down / up, not all

In the WYSIWYG editor, on long pages, pressing page down / up keys will scroll respectively to the end / beginning of the editor while in terms of user experience, scrolling down / up only a limited set of lines would seem to match better the user intent in most cases, as it would allow to skim through the page more efficiently, just as what happens with textareas.

See also this CKEditor issue: PageUp and PageDown not working