Fix some issues with `window-divider-mode'

Fix some issues with `window-divider-mode'

* lisp/frame.el (window-divider-default-places): New option.
(window-divider-mode): Remove option.
(window-divider-mode): Make it a "regular" minor mode.
(window-divider-width-valid-p): Drop frame- prefix.
(window-divider-mode-apply): New argument ENABLE.  Drop frame-
prefix.  Handle `window-divider-default-places'.
(frame--window-divider-mode-set-and-apply): Remove.
(window-divider-default-bottom-width)
(window-divider-default-right-width): Drop :group entries.
* lisp/menu-bar.el (menu-bar-bottom-and-right-window-divider)
(menu-bar-right-window-divider, menu-bar-bottom-window-divider)
(menu-bar-no-window-divider): Set `window-divider-default-places'
and call `window-divider-mode'.
* doc/emacs/frames.texi (Window Dividers): Document
`window-divider-default-places'.