API: Placeholder setting for "auto-apply preview on hover"

[maltenuhn]

Problem
Some API settings (eg those related to layout) - work best if you see a "preview" of what they do, on hover. In other tools, a common pattern for this is to apply on hover the hovered-over item in eg a list or a segment control "as if the user had already clicked on it".

Solution

• Add an (optional) key to the API (previewable, default / in absence: false)
• For segment controls only, implement a feature that "previews" the result on hover over a segment - but without the code editor updating yet.