Gutenberg: Add Screen Options equivalent

[mtias]

Surfacing this one back up as a compatibility and enhancement task. This would be the best way to provide access for _custom fields_ given we are lacking a proper API support for them. This is meant to be a quick implementation using our core APIs: plugin menu, and modal.

The options modal would combine current screen options, the tips, and the publish flow setting. It would absorb some items from the main toolbar, so that it'd end up looking like this:

[danielbachhuber]

Once this lands, we should make sure to address #3228 (even if it's just to close the issue because Custom Fields have returned).

[bkjproductions]

I would like to understand this better. Would there be a list of Custom Fields (key/value for each) in this Options modal?
Or is the purpose of the Options modal simply to show a checkbox/button to then allow another section of the interface to show the Custom Fields (key/value for each) at the bottom of the page like it does in the Classic Editor?

[youknowriad]

Or is the purpose of the Options modal simply to show a checkbox/button to then allow another section of the interface to show the Custom Fields (key/value for each) at the bottom of the page like it does in the Classic Editor?

More the second point but this is tracked in #3228