bug#34715: 26.1; (1) Add `clone-frame', (2) bind it to `C-x 5 2'

[Drew Adams]

Enhancement request:

1. Add a `clone-frame' command such as this one, which is similar to
   what I've been using:

(defun clone-frame (&optional frame no-clone)
  "Make a new frame with the same parameters as FRAME.
With a prefix arg, don't clone - just call `make-frame-command'.

FRAME defaults to the selected frame.  The frame is created on the
same terminal as FRAME.  If the terminal is a text-only terminal then
also select the new frame."
  (interactive "i\nP")
  (if no-clone
      (make-frame-command)
    (let* ((default-frame-alist  (frame-parameters frame))
           (new-fr               (make-frame)))
      (unless (display-graphic-p) (select-frame new-fr))
      new-fr)))

2. Use it, not `make-frame-command', as the binding of `C-x 5 2'.

   Why change the default behavior of `C-x 5 2'?  If I want the buffer
   of the selected window shown in another frame then I typically want
   that frame to have the same parameters.

   It's true that I often have only one window in the selected frame,
   which is not typical of other users, and in that case it perhaps
   makes even more sense that I want `C-x 5 2' to duplicate the frame's
   parameters.  But I think that most users are likely to prefer, as the
   default behavior, keeping the same parameters.  (`C-u' gives
   `make-frame-command', i.e., the frame parameters are not cloned.)  An
   open question, I guess.

3. BTW, I think it would be good to add this to the doc string of
   `make-frame-command':

   Return the new frame.

In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor `Microsoft Corp.', version 10.0.17134
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''