Block.getInputValue returns default on Cancelled.
Prevent error if _complete has been called.
getInputValue returns default if input is None.
value-changed might indicate a variable change.
self._variables should be a set.
Allow controls_wait time to be updated at runtime.

- Value can be a number or a variable / expression.
- Updates are performed relative to started time.
Handle code display scrolling properly.
Fixes for controls_wait.