* calendar/todos.el: Implement saving top priority items buffers,

* calendar/todos.el: Implement saving top priority items buffers,
improve implementation of item filtering commands and
infrastucture, and fix two bugs.
(todos-prefix-overlays): Use todos-top-priority face only in Todos
mode.
(todos-multiple-filter-files): Get file-truename of Todos file.
(todos-filter-items): Pass a list of files from the caller instead
of building it here, and move handling of cancelled file selection
dialog to callers.  Correct omission of file-wide default number
of top priorities.
(todos-filtered-buffer-name): Treat file-list argument only as a list.
(todos-find-item, todos-check-top-priorities)
(todos-top-priorities-filename)
(todos-save-top-priorities-buffer): New functions.
(todos-save): Use todos-save-top-priorities-buffer.
(todos-jump-to-item): Refactor and use todos-find-item.
(todos-top-priorities): Simplify semantics of prefix argument.
Check if top priorities file exists and if so, visit it, reporting
whether it is up to date.  Add optional argument and use it to
handle multiple files.
(todos-diary-items, todos-regexp-items): Add optional argument and
use it to handle multiple files.
(todos-top-priorities-multifile, todos-diary-items-multifile)
(todos-regexp-items-multifile): Refactor, using corresponding
non-multifile command.
(auto-mode-alist): Add pattern for top priorities files to visit
these in Todos Filtered Items mode.