Allow basic configuration for the default importer

Expose a way to configure the basic importer from a contribution file and service to invoke it.

• the implementation of the DocumentModelFactory used and the folderish/leaf type it creates
  ( if no implementation is provided DefaultDocumentModelFactory should be used, with "Folder" and "File" as document types)
• the implementation of the SourceNode used.
  (if no implementation is provided the FileSourceNode should be used).