Backport Gutenberg data API store registration and access

[IreneStr]

Current situation

The Gutenberg data API has an option to register your own store. We'd like to use this to add our own state with the data we need for our sidebar. However, this API is only available in Gutenberg, while we'll move to the sidebar in the classic editor as well.

Ideal situation

Eventually, WordPress should backport the data API to the classic editor to have the same functionality.

Temporary solution

However, there is currently no backporting of this API in WordPress. Therefore we should create the backporting ourselves.

What does it look like in Gutenberg?

Gutenberg's registerStore expects two arguments: a name, and an object containing a reducer function, an actions object, a selectors object, and a resolvers object. See https://github.com/WordPress/gutenberg/blob/master/data/README.md .

What needs to happen?

We should make sure that we can use and register a store in the classic editor exactly the same way as in Gutenberg.

Please branch from feature/content-analysis-in-sidebar

[atimmer]

Riad suggested to take a look at https://github.com/youknowriad/standalone-gutenberg to see how to make this possible.