lisp/desktop.el: Move code related to saving frames to frameset.el.

lisp/desktop.el: Move code related to saving frames to frameset.el.
Require frameset.
(desktop-restore-frames): Doc fix.
(desktop-restore-reuses-frames): Rename from
desktop-restoring-reuses-frames.
(desktop-saved-frameset): Rename from desktop-saved-frame-states.
(desktop-clear): Clear frames too.
(desktop-filter-parameters-alist): Set from frameset-filter-alist.
(desktop--filter-tty*, desktop-save, desktop-read):
Use frameset functions.
(desktop-before-saving-frames-functions, desktop--filter-*-color)
(desktop--filter-minibuffer, desktop--filter-restore-desktop-parm)
(desktop--filter-save-desktop-parm, desktop--filter-iconified-position)
(desktop-restore-in-original-display-p, desktop--filter-frame-parms)
(desktop--process-minibuffer-frames, desktop-save-frames)
(desktop--reuse-list, desktop--compute-pos, desktop--move-onscreen)
(desktop--find-frame, desktop--select-frame, desktop--make-frame)
(desktop--sort-states, desktop-restoring-frames-p)
(desktop-restore-frames): Remove.  Most code moved to frameset.el.
(desktop-restoring-frameset-p, desktop-restore-frameset)
(desktop--check-dont-save, desktop-save-frameset): New functions.
(desktop--app-id): New constant.
(desktop-first-buffer, desktop-buffer-ok-count)
(desktop-buffer-fail-count): Move before first use.
lisp/frameset.el: New file.