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

* calendar/todos.el: Extend and improve item filtering and handling of
saved files of filtered items.
(todos-show-first): Add choices for diary and regexp items.
(todos-absolute-file-name): Handle file names for diary and regexp
items.
(todos-filter-items): New function abstracted from item filtering
commands, with handling of saved files of diary and regexp items.
(todos-filter-items-1): Rename from todos-filter-items.
(todos-filtered-buffer-name): Remove, since functionality is now
incorporated into new function todos-filter-items.
(todos-filtered-items-buffer, todos-top-priorities-buffer)
(todos-diary-items-buffer, todos-regexp-items-buffer):
Make defconsts instead of defcustoms.
(todos-find-item): Restrict calculation and comparison of
priorities to top priority items, in order to handle diary and
regexp items.
(todos-check-filtered-items-file): Rename from
todos-check-top-priorities and adjust message and callers.
(todos-filter-items-filename): Rename from
todos-top-priorities-filename, adjusting caller and extending to
files of filtered diary and regexp items.
(todos-save-filtered-items-buffer): Rename from
todos-save-top-priorities-buffer and adjust caller and uses.  Use
write-file instead of write-region and use existing file name if
previously saved.
(todos-show): Handle initially displaying saved file of filtered
diary or regexp items.
(todos-top-priorities, todos-top-priorities-multifile)
(todos-diary-items, todos-diary-items-multifile)
(todos-regexp-items, todos-regexp-items-multifile): Use new
function todos-filter-items.
(auto-mode-alist): Extend regexp for files assigned to
todos-filtered-items-mode to handle saved diary and regexp items.