Define format of the generic compound document

The generic compound document is defined by:

• a new facet CompoundDocument
• a new document type CompoundDocument which use the above facet
• a new document type CompoundDocumentFolder which is a Folderish doctype

For now, there is no related schema for the facet/doctype.
Authorized subtypes for CompoundDocument and CompoundDocumentFolder are File, Picture, Video, Audio and CompoundDocumentFolder

Acceptance tests:
As a user, I can create a new CompoundDocument document through the REST API and see that:

• the returned document has the CompoundDocument facet
• the authorized subtypes are File, Picture, Video, Audio and CompoundDocumentFolder

As a user, I can create a new CompoundDocumentFolder inside an existing CompoundDocument document.