Todo mode bug fixes and new features.

Todo mode bug fixes and new features.

* calendar/todo-mode.el: Bug fixes and new features.
(todo-toggle-item-highlighting): Use eval-and-compile instead of
eval-when-compile.
(todo-move-category): Allow choosing a non-existing todo file to
move the category to, and create that file.
(todo-default-priority): New user option.
(todo-set-item-priority): Use it.
(todo-desktop-save-buffer, todo-restore-desktop-buffer): New functions.
(desktop-restore-file-buffer): Declare.
(desktop-buffer-mode-handlers): Add todo-restore-desktop-buffer.
(todo-modes-set-2): Locally set desktop-save-buffer to
todo-desktop-save-buffer.
(todo-mode, todo-archive-mode, todo-filtered-items-mode)
(auto-mode-alist): Add autoload cookie.

Fixes: debbugs:15225