lisp/desktop.el: Require 'cl-lib.

lisp/desktop.el: Require 'cl-lib.
(desktop-before-saving-frames-functions): New hook.
(desktop--process-minibuffer-frames): Set desktop-mini parameter only
for frames being saved.  Rename from desktop--save-minibuffer-frames.
(desktop-save-frames): Run hook desktop-before-saving-frames-functions.
Do not save frames with non-nil `desktop-dont-save' parameter.  Filter
out deleted frames.
(desktop--find-frame): Use cl-find-if.
(desktop--select-frame): Use cl-(first|second|third) to access values
of desktop-mini.
(desktop--make-frame): Use cl-delete-if.
(desktop--sort-states): Fix sorting of minibuffer-owning frames.
(desktop-restore-frames): Use cl-(first|second|third) to access values
of desktop-mini.  Look for visible frame at the end, not while
restoring frames.