[BUGFIX] Subfolders must use the identifier as identifier

Currently subfolders don't use the identifier correctly as the name is
used instead. This doesn't really matter if you got a folder structure
anyway but dealing with different identifiers leads to troubles.

createFolderObject() will create a new Folder object which then has the
identifier set wrong.

Change-Id: I04629ffc97766557abc0920f1dbe56f1453d76ac
Fixes: #46568
Releases: 6.1,6.0
Reviewed-on: https://review.typo3.org/19198
Reviewed-by: Tom Ruether
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer