Groupfolders overlay icon for windows explorer

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### Feature description

It would be nice t o give groupfolders an overlay icon to distinguish them from normal Nextcloud folders in Windows explorer. Especially people who are not tech-savvy easily get confused about the different types of folders in Nextcloud. It often happens that they accidently move or delete files in groupfolders. That's why the overlay icon would help.

Cc @tobiasKaminsky @mgallien