Document and fix some bugs with side windows

Document and fix some bugs with side windows

Add a documentation for side windows and fix some bugs
found when testing their behavior.  Also add a new window
parameter `no-delete-other-window', a new `display-buffer'
alist member called `window-parameters', and functions to
toggle and reverse side windows on a frame.  Add new function
`window-swap-states' to exchange states of two live windows.

* lisp/window.el (display-buffer-in-atom-window): Use
`split-window-no-error'.
(window-sides-vertical): Maybe change layouts when setting this
variable.
(window-sides-reversed): New option.
(window-sides-slots): Rewrite doc-string and help echoes.
(window-sides-shown): New buffer-local variable set when showing
a buffer in a side window.
(window--sides-inhibit-check): New variable.
(window--sides-reverse-on-frame-p, window-toggle-side-windows)
(window--sides-reverse-all, window--sides-reverse-frame)
(window--sides-reverse-side, window--sides-reverse)
(window--sides-verticalize-frame, window--sides-verticalize)
(window--sides-check-failed): New functions.
(window--side-window-p): Remove function.
(window--major-non-side-window): Rename to `window-main-window',
adjust callers, rewrite doc-string.
(window--major-side-window): Rename to
`window--make-major-side-window-next-to', adjust caller, fix
doc-string.
(display-buffer-in-major-side-window): Rename to
`window--make-major-side-window', adjust
caller, rewrite doc-string.  Make `window-side' and
`window-slot' parameters persistent (Bug#23858).  Don't set
`delete-window' parameter.  Add `preserve-size' entry to ALIST.
(delete-side-window): Remove function.
(display-buffer-in-side-window): Fix doc-string.  Don't set
`delete-window' parameter.  Add `preserve-size' entry to ALIST.
(window--side-check): Rename to window--sides-check.  Rewrite
completely.  Adjust caller.
(window-resize-no-error): Don't describe PIXELWISE argument.
(adjust-window-trailing-edge): Fix bug that disallowed
re-enlarging windows that were too small.
(window-deletable-p): Don't tell that a minibuffer window on a
non-minibuffer-only frame can be deleted.  Fix doc-string.
(delete-window): Handle deleting a side window here (the
`delete-window' parameter is no more set for side windows).
(delete-other-windows): Handle ‘no-delete-other-window'
parameter. Don't treat side windows separately (see discussion
of Bug#24368) but keep optimization that makes the main window
the root window of its frame.
(switch-to-prev-buffer, switch-to-next-buffer): Handle side
windows and buffers shown in side windows separately.
(split-window-no-error): New function.
(window--state-get-1): Use right buffer when storing window
point and start positions and WRITABLE is nil (Bug#24368).
(window--state-put-1): Fix handling of `window-combination-limit'.
Use `split-window-no-error'.
(window--state-put-2): Try to restore windows with preserved
size to their original size.  Fix bug where a fixed window's width
was not preserved.
(window-state-put): When reducing an internal window to a live
one, don't choose a side window.
(window-swap-states): New function.
(window-splittable-p): Don't call `window--side-window-p'.
(window--display-buffer): Handle `window-parameters' ALIST entry.
Minor rewrite.
(display-buffer): Mention `window-parameters' entry in
doc-string.
(display-buffer-at-bottom): Call `split-window-no-error'.

* doc/lispref/elisp.texi (Top): New section "Side Windows".
* doc/lispref/windows.texi (Deleting Windows): Fix descriptions
of `delete-window' and `delete-other-windows' wrt window
parameters and side windows.
(Display Action Functions): Mention `window-parameters' ALIST
entry.
(Side Windows): New section (Bug#18170).
(Window Configurations): Describe new function `window-swap-states'.
(Window Parameters): Say that functions may behave specially when
their homonymous window parameter has been set.  Mention new
parameter `no-delete-other-window'.  Add cross reference for
`window-side' and `window-slot' parameters.