Rewrite handling of side and atomic windows.

Rewrite handling of side and atomic windows.

* window.el (display-buffer-in-atom-window, window--major-non-side-window)
(window--major-side-window, display-buffer-in-major-side-window)
(delete-side-window, display-buffer-in-side-window): New
functions.
(window--side-check, window-deletable-p, delete-window)
(delete-other-windows, split-window): Handle side windows and
atomic windows appropriately.
(window--display-buffer): Call display-buffer-record-window also
when the window buffer did not change.