Add code for "preserving" window sizes.

Add code for "preserving" window sizes.

* frame.c (frame_windows_min_size): New argument IGNORE.
(adjust_frame_size): When called from change_frame_size call
frame_windows_min_size with IGNORE Qt so we can ignore size
restrictions.

* dired.el (dired-pop-to-buffer): Call fit-window-to-buffer with
`preserve-size' t.
(dired-mark-pop-up): Preserve size of window showing marked
files.
* electric.el (Electric-pop-up-window):
* help.el (resize-temp-buffer-window): Call fit-window-to-buffer
with `preserve-size' t.
* minibuffer.el (minibuffer-completion-help): Use
`resize-temp-buffer-window' instead of `fit-window-to-buffer'
(Bug#19355).  Preserve size of completions window.
* register.el (register-preview): Preserve size of register
preview window.
* tmm.el (tmm-add-prompt): Call fit-window-to-buffer
with `preserve-size' t (Bug#1291).
* window.el (with-displayed-buffer-window): Add calls to
`window-preserve-size'.
(window-min-pixel-size, window--preservable-size)
(window-preserve-size, window-preserved-size)
(window--preserve-size, window--min-size-ignore-p): New
functions.
(window-min-size, window-min-delta, window--resizable)
(window--resize-this-window, split-window-below)
(split-window-right): Amend doc-string.
(adjust-window-trailing-edge): Handle preserving window
sizes.  Signal user-error instead of an error when there's no
window above or below.
(window--min-size-1, window-sizable, window--size-fixed-1)
(window-size-fixed-p, window--min-delta-1)
(frame-windows-min-size, window--max-delta-1, window-resize)
(window--resize-child-windows, window--resize-siblings)
(enlarge-window, shrink-window, split-window): Handle preserving
window sizes.
(window--state-put-2): Handle horizontal scroll bars.
(window--display-buffer): Call `preserve-size' if asked for.
(display-buffer): Mention `preserve-size' alist member in
doc-string.
(fit-window-to-buffer): New argument PRESERVE-SIZE.