The document parent is not updated when a file is moved

A file can have multiple parent folders, which are specified using tags, because the document used to describe a file can have only one real parent. The File Manager should set though the document parent to the first parent folder so that:

• the document hierarchy is still displayed nicely outside of the File Manager
• we can detect more easily orphan files

The parent is set when a file is created under a folder but is not updated when the file is moved.