* todos.el: Improve toggling of item numbering and headers.

* todos.el: Improve toggling of item numbering and headers.
(todos-prefix-overlays): Don't reset item numbering for
done items unless in todos-mode.
(todos-archive-mode-map): Add bindings for todos-toggle-item-header.
(todos-toggle-prefix-numbers): Reimplement without using
todos-reset-prefix and make it work in Todos Archive mode and
Todos Filtered Items mode.
(todos-toggle-item-header): Make it work in Todos Archive mode and
Todos Filtered Items mode.