Code improvement to make pylint happier.
Implement convention of super-messaging handlers.
Enforce new "machine modifier" protocal.

Backtracking is a "machine modifier" so it must be before the machine
class it modifies in the mro.
Add more super() calls for machine modifiers.
Rename no_transition() to on_no_transition().

This is consistent with the naming convention of handlers.
Separate Backtracking from StateMachine better.

All special handling for Backtracking has been removed in favor of more
general notifications.