TodoList - No functional changes. Improvements.

- Adding 2 new cell methods: markTaskComplete and markTaskIncomplete so
that we can reuse the code that updates the strikethrough appearance of
a task on a cell.
- Moving the animation when swiping a cell left and right so that it’s
done as part of the hideCircleButtons and showCircleButtons
implementation. This makes sense since the cell is better aware of the
button sizes and the layout so can keep all the x and y hardcoded
values in one place.
- Updating the MOVE class to use latest syntax on accessing dictionary
values.
- Submitting .pbxproj which should’ve been committed previously as it
contains updates related to the new MOVE class.