[BUGFIX] Moving folders fails

On moving existing folders inside another existing folder in the
file-module, the following exception is thrown:

1314085991: uid of Storage has to be numeric.

The reason is obvious, since the submitted storage key contains
the storage object instead of the UID of the storage.

Change-Id: Ia6f6af5252880a7bde8a537bc9e343c392eadbf5
Fixes: #52845
Releases: 6.0, 6.1, 6.2
Reviewed-on: https://review.typo3.org/24792
Reviewed-by: Xavier Perseguers
Reviewed-by: Frans Saris
Tested-by: Frans Saris
Reviewed-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind