* calendar/todos.el: Further comment revision.

* calendar/todos.el: Further comment revision.
(todos-sorted-column): Change default value.
(todos-item-start): Handle empty category (needed in
todos-filter-items).
(todos-read-date): Don't use calendar-read; make code cleaner.
(todos-multiple-filter-files): Rename this variable from
todos-multiple-files and adjust users.
(todos-multiple-filter-files-widget): Rename from
todos-multiple-files-widget and adjust users.
(todos-multiple-filter-files): Rename this function from
todos-multiple-files and adjust callers.
(todos-filter-items): Remove unused code.
(todos-insert-category-line): Add space so highlighting of last
column is consistent with the others; adjust display of column
highlighting.
(todos-menu): Remove obsolete entry.
(todos-categories-mode-map): Add new bindings.
(todos-display-categories-alphabetically-or-by-priority): New command.
(todos-display-categories-sorted-by-todo)
(todos-display-categories-sorted-by-diary)
(todos-display-categories-sorted-by-done)
(todos-display-categories-sorted-by-archived): Restore and fix
implementation.