getEntriesByCollection available in previews

I do not see this feature request opened, but may have missed something. It is similar to the request on the answers.netlify.com site: https://answers.netlify.com/t/access-entries-data-from-netlify-cms-events/15985.

**Is your feature request related to a problem? Please describe.**
Producing a proper preview in the preview pane when an item contains a reference to another collection (relation widget) is currently not possible, as access to the external collection isn't available.

**Describe the solution you'd like**

**Describe alternatives you've considered**
For creating previews, it would be nice to have access to other collections, as the relationship field points to them, and to create a decent preview, access to the collection referenced by the relationship field is crucial. Something like:

`const otherEntries = getEntriesByCollection('some-collection')`

**Additional context**
I would use this in a renderer as follows:

var AtlasLandingPreview = createClass({

render: function() {

const entry = this.props.entry;

const getAsset = this.props.getAsset;

const widgetFor = this.props.widgetFor;

const widgetsFor = this.props.widgetsFor;

const allNewsArticles = getEntriesByCollection('news');

return h('ul', {className: 'content'},

widgetsFor('articles').map(function(article,i) {

// a search of the allNewArticles collection would be performed here, looking for a matching article.uuid in my case

var foundArticle = findArticleByUUID(article);

// other fields like foundArticle.thumb, foundArticle.date, etc could be accessed here

return h('li', {}, foundArticle.title); // note: my articles aren't stored by title, but by UUID

})

);

}

});

(Of course some of that code is just sudo-code)