Moves iter forward by one display (wrapped) line. A display line is
different from a paragraph. Paragraphs are separated by newlines or
other paragraph separator characters. Display lines are created by
line-wrapping a paragraph. If wrapping is turned off, display lines and
paragraphs will be the same. Display lines are divided differently for
each view, since they depend on the view's width; paragraphs are the same
in all view, since they depend on the contents of the W(TextBuffer).