Editor: Stop exposing internal components starting with underscore

[gziolo]

We iniitally exposed wp.editor._BlockSettingsMenuFirstItem in #7199:

https://github.com/WordPress/gutenberg/blob/master/packages/editor/src/components/index.js#L87

#7895 is going to introduce another component which follows the same approach.

We should refactor code to stop exposing components that are not meant for public consumption.

[gziolo]

@aduth @youknowriad any ideas on how to attack it? Can we move those slots to edit-post and inject to BlockSettingsMenu similar to what we do with publish panels?

[aduth]

The introduction in #7199 was a bit of a judgment call and in some ways a stop-gap solution to provide some level of prioritization to menu items. If we want to surface priority: We should do so. If we don't: Then _ prefixing is fine (see also mention of precedent).

[gziolo]

@aduth, it seems like we should rename them to begin with __Unstable*, what do you think?

[aduth]

Yes, I recall mentioning it recently as well in some pull request. I think it would be a good idea.

[gziolo]

When reviewing #15195 I discovered that those components listed in this issue are marked now as experimental. We can close this issue now.