Moved default virtual method back to header file.
Improving Canvas functions. Adding further acknowledgements.

1. Adding scale parameter to draw_poly and draw_stroke.
2. Adding new text mode for automatic scrolling in text ports.
3. Adding form-feed ('\f') handling in text ports.
4. Adding draw stroke to canvas script operations.
Improving code description. More details on descriptor numberings.
Moved filtering code to on_event() handler. Added on_change() method.
Documenting the event code usage.
Adding a new variant of sample() to give the same interface as AnalogPin.