#7 Can't unset a due date

[Gus Heck]

Steps to reproduce
1. Download and run a fresh copy of Nato
2. Right Click on "My Default Task Model" to produce a context menu
3. Select add task
4. Enter a name for the new task
5. Select the task in the left hand tree UI.
6. Click the calendar widget in the upper right pannel
7. Select a date
8. Observe the Due date applied.
9. Left and right click the date and the yellow box to the left of the "Due" label.
10. Click on the calendar widget.

Observed behavior
The Due date is set and displayed, but no UI widget appears to allow the due date to be removed.

Expected behavior
There should be a way to remove the due date entirely, either within the calendar widget or associated with the date display