Fix the order of document type contributions in the generated extensions.xml

Steps to reproduce:

1. define a document type named ECMFolder which extends Folder and make it available for Folder and Workspace in the Container types
2. define another document type named BBBDOCUMENT which extends File and select ECMFolder in the Container types
3. Deploy the project to a Nuxeo instance
4. Create a ECMFolder document
5. Click on New and observe you cannot create a BBBDOCUMENT

The problem is due to the fact that the definition of ECMFolder is written after the one of BBBDOCUMENT which holds the "container types" information.

TODO:

• Add a validation warning when a document type is said it can be created in a container, but not referenced in the container as an accepted child type:

  This document type is configured to be created in the [doctypeName] document type, but not referenced as an accepted child type in this document type.

• Add a validation warning when a document type is put as an accepted child type in a container, but the container is not referenced in the doctype:

  This document type is referenced as an accepted child type in the [containerName] document type, but [containerName] is not added in the container types.

  Both validation warnings apply to the child document type, not its container.

NOTES

• Changing the order in which contributions are generated is considered out of scope, only validation would be provided