TodoList - Stop swipe left

If a cell is shifted over to the left (after a swipe gesture) and the
user taps it, we now stop the task from being marked as completed.
- Adding a new boolean property slidOver which is set whenever we
show/hide the delete/change priority buttons.
- The TableVC checks this property in the didSelectRowAtIndexPath and
if it returns true, then we ignore the user’s tap (aka selection).