Make some cleanup around Section and publishing

The following points need to be improved:

• the facet MasterPublishSpace and PublishSpace are respectively associated to SectionRoot and Section in nuxeo-platform-webapp-types whereas it should be defined in nuxeo-core
• the Unpublish action is statically associated to SectionRoot and Section: if someone defines a new document type that inherits Section, Unpublish won't be available => we should use a filter on PublishSpace facet instead
• it's not possible to publish under SectionRoot, so Unpublish should not be displayed for this document type