* calendar/todos.el: Implement extended category completions.

* calendar/todos.el: Implement extended category completions.
(todos-default-todos-file): Change default value to correct custom
type.
(todos-category-completions-files): New defcustom.
(todos-reevaluate-category-completions-files-defcustom)
(todos-absolute-file-name, todos-category-completions): New defuns.
(todos-reevaluate-filelist-defcustoms):
Add todos-category-completions-files and
todos-reevaluate-category-completions-files-defcustom.
(todos-allcats-file, todos-all-categories-alist)
(todos-jump-to-category-other-file, todos-jump-to-any-category)
(todos-move-item-to-file): Remove (todos-all-categories-alist both
as defvar and as defun).
(todos-read-category): Rewrite using extended category completions
and better handling of completion against existing or non-existing
categories.
(todos-validate-name): Correct doc string; use local instead of
dynamic variables in function body.
(todos-insert-category-line): Adjust use of todos-jump-to-category
to new argument list.
(todos-key-bindings, todos-menu): Delete entries for removed
commands.
(todos-show): Get absolute name of todos-default-todos-file;
add `nowarn' argument to find-file-noselect; adjust use of
todos-add-category to new argument list.
(todos-jump-to-category): Rewrite using extended category
completions; reverse argument list, adapting FILE argument to new
completion mechanism; add autoload cookie; call todos-show if
there are no Todos files.
(todos-add-category): Rewrite using extended category completions;
add FILE argument to accommodate new completion mechanism.
(todos-merge-category): Adapt to extended category completions,
adding prefix argument.
(todos-insert-item): Adapt to extended category completions; use
todos-read-category instead of todos-jump-to-category in prefix
argument calls; call todos-show if there are no Todos files; if
item is inserted at end of category, recenter to ensure the items
above it are displayed in the window.
(todos-move-item): Adapt to extended category completions, making
argument prefix argument; allow moving within category (for the
sake of code simplification); add `nowarn' argument to
find-file-noselect.
(todos-archive-done-item): Prevent error when attempting to
archive not-done todo item; adjust use of todos-add-category to
new argument list.