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.
(ses-header-line-menu): Fix missing variable rename for header-row.
(ses-cell-size): Remove.
(ses-make-cell): New function.
(ses-cell, ses-insert-row, ses-insert-column): Use it.
(ses-calculate-cell): Remove unused var `symbol'.
(ses-narrowed-p): New function.
(ses-goto-data, undo-more, ses-reconstruct-all): Use it.
(ses-initial-file-trailer): Change ;;; to ;; for local vars.
(ses-load, ses-reconstruct-all): Adjust string search accordingly.
(ses-setup): Use restore-buffer-modified-p.
(ses-cleanup): Remove unused var `end'.
(ses-header-string-left-offset): Remove.
(ses-create-header-string): Adjust to new behavior of `align-to'.
Truncate excessively large fields to preserve alignment.
(ses-reconstruct-all): Remove unused var `refs'.
(ses-read-cell-printer): Remove unused var `prompt'.
(ses-delete-row): Remove unused var `pos'.
(ses-delete-column): Remove unused var `new'.
defconst's for the three maps. Add menus. Use "ses--" prefixes for
buffer-local variables. Use (point-min) instead of 1, even when we know the
buffer is unnarrowed. New function ses-read-number duplicates some code
from interactive "N" spec.
