Comment 2 for bug 1605905

Per https://developer.gnome.org/gtk3/stable/gtk3-Stock-Items.html#GTK-STOCK-DIRECTORY:CAPS it looks like we should actually use the named icon "folder" instead of "gtk-directory".

In the past, I have elected to use "applications-other/gnome-other" since it represented a generic applications folder. At this point, it would probably be an improvement to just use an actual folder.