Upgrade LinearCellDemo and add a PythonicAPI version

Changes are:

1. A better colour choice, making it easier to distingiush fromt and back surfaces (I hope).
2. Enlarging the point ID numbers.
3. Rewrote/upgraded the C++ and Python versions.
4. Added an example for the new PythonicAPI - this example uses the vtkTextWidget and adds borders.
5. Updated the documentation to help in identifying the back face.