TodoList - Add delete button on swipe left

When swiping left on a list item that is completed, we will now show a
delete button instead of the 3 change priority color buttons.

- Adding new cell methods showDeleteButton and hideDeleteButton. The
delete button is custom, consists of red circle button with white text
on a label.
- Updated the TableVC swipeCell method to make call these methods after
a swipe, taking into account that only completed tasks should result in
showing the delete button.