More granular options for pushing local styles to global styles

[jameskoster]

A follow-up to #44361.

After we make it possible to push all local block styles to global styles, it could be good to explore how we can add a little more granularity to the feature.

There will be situations in which you heavily customise a block locally and want to push some of those changes upstream, but keep some local.

Per-panel pushing would afford more granularity, and per-control pushing would take things to the extreme. A rough mockup meant only to communicate the idea:

In order to implement this, we'll need to design a pattern for the UI which indicates when the styles within a local panel have departed from global styles. A dot marker was suggested in 44361 but ruled out due to "unread-dot fatigue". Let's explore alternatives, and discuss whether we need both per-panel and per-control pushing.