Undelete deleted frames.

Undelete deleted frames.

* lisp/frame.el (undelete-frame): New command.
(undelete-frame--handle-delete-frame): New auxiliary function.
(undelete-frame--deleted-frames): New auxiliary variables.
(undelete-frame-mode): New minor mode.
(ctl-x-5-map): Bind the new command.

* etc/NEWS: Document the new command and minor mode.

* src/frame.c (Fdelete_frame): Update docstring, and mention the
minor mode.

* lisp/menu-bar.el (menu-bar-file-menu): Add an entry for the
new command.

* doc/emacs/frames.tex (Frame Commands): Document the new command
and minor mode.

See bug#51883.