Expose Document Type icons for use with `icon` properties

In Designer you can set an icon for a Button, for example. The icon set is made of OOTB icons and custom icons that you upload. It would be cool if the Document Type icons were exposed here as well, so I don't have to upload and maintain them in two places.

For example I have a document type icon named "case.svg" for the Case document type. I would like to use this icon in `nuxeo-home.html` in the heading for a `nuxeo-card` that has a list of Cases. As of today I need to upload another copy of the icon using the "Import a custom icon" feature.