Improve accessibility of window dividers. (Bug#20183)

Improve accessibility of window dividers.  (Bug#20183)

* lisp/faces.el (window-divider)
(window-divider-first-pixel, window-divider-last-pixel): Change
membership from `frames' to `window-divider' customization group.
* lisp/frame.el (window-divider): New customization group.
(window-divider-mode): New minor mode.
(window-divider-default-bottom-width)
(window-divider-default-right-width): New options.
(frame--window-divider-previous-mode): New variable.
(frame-window-divider-width-valid-p)
(frame--window-divider-mode-apply)
(frame--window-divider-mode-set-and-apply): New functions.
* lisp/menu-bar.el (menu-bar-options-save): Save
window-divider-mode settings.
(menu-bar-window-divider-customize)
(menu-bar-bottom-and-right-window-divider)
(menu-bar-right-window-divider, menu-bar-bottom-window-divider)
(menu-bar-no-window-divider): New functions.
(menu-bar-showhide-window-divider-menu): New variable.
(menu-bar-showhide-menu): Show/hide window divider menu.
* lisp/mouse.el (mouse-split-window-vertically)
(mouse-split-window-horizontally): Replace `error' by
`user-error'.  Bind `window-combination-resize' to nil.
(top-level): Add/reorder mouse key bindings on mode- and
vertical-line.