* ses.el (defadvice undo-more): Delete this defadvice. The undo

* ses.el (defadvice undo-more): Delete this defadvice.  The undo
	overrides will now be done a different way.
	(ses-set-parameter): Reapply this function for undo.
	(ses-set-header-row): Reconstruct header row during undo.
	(ses-widen): New function.
	(ses-goto-data, ses-reconstruct-all): Use new function.
	(ses-command-hook): Widen buffer during undo, before unupdating
	the cells.
	(ses-insert-row, ses-delete-row): Widen buffer during undo.
	(ses-load, ses-header-row): Permit empty (zero-row) spreadsheets.
	(ses-read-cell): Avoid stupid warning for RET RET on a cell whose
	formula hasn't been executed yet.