Make the importer easier to configure

The current importer is an hybrid between a Java Framework and a Nuxeo Service with Extension Point.

Feedback on importer is that it seems complex to most people.

We want to improve that

Target is :

• migrate to a real service / extension point system
• extract clean interface when possible
  • currently we have a of abstract class just for code sharing issues, but no interface
• allow to have more than one importer chain defined
• integrate features from the Scan importer, and XML importer

This may lead to a massive refactoring ...