Resurrect the display of subfolders for VFS sync folders

Resurrect the display of subfolders for VFS sync folders

This got removed from the settings since in that case selective sync
isn't supported. Unfortunately that's also necessary to display them to
allow encrypting folders via the context menu.

So we display the subfolders again but in the case of VFS we disable the
ability to (un)check them. They just have an icon, a name and a context
menu.

Signed-off-by: Kevin Ottens <kevin.ottens@nextcloud.com>