Further performance enhancements

[ian]

If one character in an Edit widget changes, the best case would be for a single character update to be propagated to the display. The changes that closed #1 do not achieve this ideal.

Extending the widget interface to include an optional child_modified() method could eliminate the need to render each of the parent widgets of a modified widget. Future enhancements to this interface could also pass along modifications such as "scroll n rows up" which could be passed directly to the user's terminal as an escape sequence.