Add debug event instrumentation to turtle methods.

This change adds code to report turtle motion events to a parent frame
debugger, if present.  Also, certain methods will now take time to run,
to make them easier to understand in a debugger (for example, dot will
take time to draw the dot; and scale will take time to animate the
scaling).

This change hardens and productionizes debugger code that was
developed at the Feb 14 hackathon.