Make menu bar entries only point to named functions, not lambdas

Make menu bar entries only point to named functions, not lambdas

* lisp/menu-bar.el ()
(menu-bar--display-line-numbers-mode-visual)
(menu-bar--display-line-numbers-mode-relative)
(menu-bar--display-line-numbers-mode-absolute)
(menu-bar--display-line-numbers-mode-none)
(menu-bar--visual-line-mode-enable)
(menu-bar--toggle-truncate-long-lines)
(menu-bar--wrap-long-lines-window-edge): Make lambdas into trivial
wrapper functions so that `C-h k' on the menu items work (bug#13841).
(menu-bar-showhide-line-numbers-menu)
(menu-bar-line-wrapping-menu): Use them.