* calendar/todos.el: Extend and improve handling of item editing,

* calendar/todos.el: Extend and improve handling of item editing,
especially of date/time header.
(todos-month-name-array, todos-month-abbrev-array): New defconsts.
(todos-date-pattern): Use explicitly numbered groups.
(todos-read-date): Optionally read and return just one of the date
string components year, month or monthname, day.
(todos-key-bindings): Add bindings for new item header editing
commands.
(todos-edit-item): Exclude date/time header from minibuffer by
default; include it by passing a prefix argument.
(todos-edit-item-header-1): New function containing the guts of
the commands for editing item date/time headers.
(todos-edit-item-header): Use it.  Condition editing of time
string on value of `todos-always-add-time-string'.
(todos-edit-item-date-from-calendar): Use todos-edit-item-header-1.
(todos-edit-item-date-to-today): Rename from
todos-edit-item-date-is-today and use todos-edit-item-header-1.
(todos-edit-item-date): Remove.
(todos-edit-item-date-day-name, todos-edit-item-date-year)
(todos-edit-item-date-month, todos-edit-item-date-day): New commands.