Folders should be displayed in thumbnail view as an icon

Steps to reproduce

1. Open assets app
2. Open thumbnail view 

Expected results

Only assets are shown

Actual results

Folders are also shown, which don't show an icon so lots of empty thumbnails appear

Workaround

Development notes

This is happening because when info.magnolia.ui.contentapp.browser.ThumbnailView instantiates info.magnolia.ui.vaadin.layout.LazyThumbnailLayout, it is making use of the configured info.magnolia.ui.contentapp.JcrDataProvider, which retrieves the configuration from the datasource definition, allowedNodeTypes property (mgnl:folder and mgnl:asset)
On the contrary, in the old assets app, info.magnolia.ui.workbench.thumbnail.JcrThumbnailContainer was used, which includes a specific constraint not to retrieve items of type mgnl:folder

Final solution

Folder icons is use as preview for folders.