Avoid making todo-mode buffers manually editable

Avoid making todo-mode buffers manually editable

* lisp/calendar/todo-mode.el (todo-add-category)
(todo-move-category, todo-edit-item--header)
(todo-set-item-priority, todo-move-item, todo-item-undone)
(todo-archive-done-item, todo-set-category-number): Restrict the
scope of nil buffer-read-only to the function calls that change
buffer text, thereby preventing todo mode buffers from becoming
manually editable and hence possibly corrupted when the minibuffer
is in use.