Add group separation support for mod/folder module (internal plugin).

Course configuration:

    Group mode: Separate groups

    Force group mode: yes

Currently if you add a folder module to the course, all uploaded files and created directories become visible to all users of all groups.

My proposal is to add to the folder module a group separation configuration, so each group can only view its own files and directories.

In folder settings the option to configure this separation doesn't exist:

But exists for the mod/forum:

Note: In this issue "Components/s" field, the Folder isn't there, so I chose Groups and Unknown.