Upgrade LinearCellDemo and add a PythonicAPI version
Changes are:
- A better colour choice, making it easier to distingiush fromt and back surfaces (I hope).
- Enlarging the point ID numbers.
- Rewrote/upgraded the C++ and Python versions.
- Added an example for the new PythonicAPI - this example uses the vtkTextWidget and adds borders.
- Updated the documentation to help in identifying the back face.