Ability to get the same behavior as the hidden widget in a custom widget

**Is your feature request related to a problem? Please describe.**
I would like to create a version of [my uuid widget](https://github.com/netlify/netlify-cms/issues/1407#issuecomment-448625518) that does not render a control or a preview, similar to the `hidden` widget. I need to use it in various nested settings where having a visible widget crowds up the screen too much.

However, it appears that the `hidden` widget achieves it hidden-ness by being special cased in various places:

* https://github.com/netlify/netlify-cms/blob/b1e490dce5dfaa7dceaf85d710275d67f5dae687/packages/netlify-cms-core/src/components/Editor/EditorControlPane/EditorControlPane.js#L55
* https://github.com/netlify/netlify-cms/blob/b1e490dce5dfaa7dceaf85d710275d67f5dae687/packages/netlify-cms-core/src/components/Editor/EditorControlPane/EditorControlPane.js#L27
* https://github.com/netlify/netlify-cms/blob/b1e490dce5dfaa7dceaf85d710275d67f5dae687/packages/netlify-cms-core/src/components/Editor/EditorPreviewPane/EditorPreview.js#L7
* https://github.com/netlify/netlify-cms/blob/b1e490dce5dfaa7dceaf85d710275d67f5dae687/packages/netlify-cms-core/src/components/Editor/EditorPreviewPane/EditorPreview.js#L7

... and a few more.

**Describe the solution you'd like**

That widgets can somehow opt into being hidden and receive the same treatment as the `hidden` widget.

(Although I guess that would mean that my hack to call `onChange` in `componentDidMount` wouldn't work, but hopefully we can find a way to avoid that :)

**Describe alternatives you've considered**

To just live with all those id fields being visible in my editor components.
Alternatively to use the `hidden` widget with a default function (suggested in https://github.com/netlify/netlify-cms/issues/1407), but for my use case that's presently blocked by something like https://github.com/netlify/netlify-cms/pull/1662 as default values for nested fields don't seem to work at all right now.