bpo-33664: Scroll IDLE editor text by lines (GH-7351)

bpo-33664: Scroll IDLE editor text by lines (GH-7351)

Previously, the mouse wheel and scrollbar slider moved text by a fixed
number of pixels, resulting in partial lines at the top of the editor
box. The change also applies to the shell and grep output windows,
but not to read-only text views.
(cherry picked from commit d49dbd9acc6db544ca6cb2445fe17eb0c3be4bba)

Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>