Provide content views to handle lists of documents rendering and additional filters

A "content view" is a notion to put together several configurations, mostly
already available in Nuxeo:

• query model and result provider to perform a query
• optional layout on the query model document to add filter criteria to the
  query
• results display (layout listing(s), pagination, buttons for actions on
  selected documents)

The goal of having this notion is to be able to:

• define a content view independantly from a document type (cf search center,
  DAM or search pages)
• define a content view attached to a document type to customize a folderish
  document content listing
• make it possible to configure a whole content view from studio