Add an option to show uncompleted tasks

Currently "Active Tasks" shows tasks due in the next 365 days. A common use-case is that one creates a task (e.g. on a mobile device) and does not set an explicit due date. Such tasks would not be listed under "Active Tasks". Thus create a new filter "Uncompleted Tasks" that picks up uncompleted tasks irrespective of the existence of a due date.

Closes #858 (closed) .