Detect and distinguish changes that have no visual result

[peterqliu]

Per shareshare, @almccon mentioned that some style changes (e.g. applying or undoing a default style value) don't result in a visual change, and we can flag that.

This is doable for most properties, by comparing values against a list of defaults in the style spec. As a first iteration we can just compare literal values, though expressions can also evaluate to equivalent values.

[mulloverit]

Stale and unactionable. Closing!