overview: Folder support

For some things it would be nice to group in folders. We could use the already existing /org/gnome/desktop/app-folders/folders/. The overview would then just show an icon and when tapped show the folder content. Swipe left/right would go one level up (and/or we add an explicit back action).

Shouldn't be too hard to add (I mostly didn't do so since I wanted to see where !1079 moves but as that looks stalled we can just as well add folders first.