New experimental feature to save&restore window and frame setup.

New experimental feature to save&restore window and frame setup.

* etc/NEWS: Document new Desktop option `desktop-save-windows'.

* lisp/desktop.el (desktop-save-windows): New defcustom.
  (desktop--saved-states): New var.
  (desktop--excluded-frame-parameters): New defconst.
  (desktop--filter-frame-parms, desktop--find-frame-in-display)
  (desktop--restore-windows, desktop--save-windows): New functions.
  (desktop-save): Call `desktop--save-windows'.
  (desktop-read): Call `desktop--restore-windows'.