Button to temporarily disable steps

[mvahowe]

Use cases include:

• turning of later parts of a pipeline while debugging the earlier parts
• turning of displays that are not needed right now for less scrolling

At the UI level this involves adding a toggle for a boolean value called, say, disabled to each SpecStep. This information would be added to specSteps via the update callback.

At the business level, I think we would just need to filter specSteps at the top of runCallback to return an array that only contains non-disabled steps, and then use that for subsequent processing.