Update todo-mode defcustoms in a less hideous way

Update todo-mode defcustoms in a less hideous way

* lisp/calendar/todo-mode.el (todo-reevaluate-filelist-defcustoms)
(todo-reevaluate-default-file-defcustom)
(todo-reevaluate-category-completions-files-defcustom)
(todo-reevaluate-filter-files-defcustom): Delete these functions.
(todo-update-filelist-defcustoms): New function.  This replaces
todo-reevaluate-filelist-defcustoms, using the 'custom-type'
property instead of re-evaluating the defcustoms.
(todo-add-file, todo-rename-file, todo-delete-file)
(todo-delete-category, todo-move-category)
(todo-convert-legacy-files, todo-check-file): Replace call of
todo-reevaluate-filelist-defcustoms by
todo-update-filelist-defcustoms.
(todo-show, todo-category-completions): Replace call of
todo-reevaluate-* function by use of 'custom-type' property.