Moves the given
iter backward to the next display line start.
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 views, since they depend on the contents of the `GtkTextBuffer`.