* calendar/todo-mode.el: Add handling of file deletion, both by

* calendar/todo-mode.el: Add handling of file deletion, both by
mode command and externally.  Fix various related bugs.  Clarify
Commentary and improve some documentation strings and code.
(todo-delete-file): New command.
(todo-check-file): New function.
(todo-show): Handle external deletion of the file we're trying to
show.  Replace called-interactively-p by an optional
prefix argument to avoid problematic interaction with catch form
when byte compiled (bug#14702).
(todo-quit): Handle external deletion of the archive's todo file.
Make sure the buffer that was visiting the archive file is still
live before trying to bury it.
(todo-category-completions): Handle external deletion of any
category completion files.
(todo-jump-to-category, todo-basic-insert-item): Recalculate list
of todo files, in case of external deletion.
(todo-add-file): Replace unnecessary setq by let-binding.
(todo-find-archive): Check whether there are any archives.
Replace unnecessary setq by let-binding.
(todo-archive-done-item): Use find-file-noselect to get the
archive buffer whether or not the archive already exists.  Remove
superfluous code.  Use file size instead of buffer-file-name to
check if the archive is new; if it is, update list of archives.
(todo-default-todo-file): Allow nil to be a valid value for when
there are no todo files.
(todo-reevaluate-default-file-defcustom): Use corrected definition
of todo-default-todo-file.
(todo-key-bindings-t+a+f): Add key binding for todo-delete-file.
(todo-delete-category, todo-show-categories-table)
(todo-category-number): Clarify comment.
(todo-filter-items): Clarify documentation string.
(todo-show-current-file, todo-display-as-todo-file)
(todo-reset-and-enable-done-separator): Tweak documentation string.
(todo-done-separator): Make separator length window-width, since
bug#2749 is now fixed.

Fixes: debbugs:14688