* calendar/todos.el (todos-reset-prefix): Don't remove overlays,

* calendar/todos.el (todos-reset-prefix): Don't remove overlays,
since it's superfluous and also wrongly removes items marks.
(todos-category-completions): Select current category on visiting
file, to ensure category is properly displayed if user switches to
file via a non-Todos command.
(todos-mark-category): Make implementation cleaner.
(todos-unmark-category): Improve implementation.  Get overlay
property only when there is an overlay.