Add signal to process public URL of file
By using the File Abstraction Layer (FAL), resources can be put to remote storages like Amazon S3 or WebDAV.
However, in those cases a protected URI (containing user credentials) is used for uploading which shall not be accessible for any user of course.
This change introduces the possibility to pre-process the public URL generation and allows i.e. to substitute by an alternative dispatcher URL.
Example for registering the slot: