Add Introspection API

We have a REST API but so far we don't have a way for a remote client to easily introspect the structures available on the server side :

• schemas
• document types
• widgets
• layouts

Having such an API end point does make sense :

• CMIS has this (at meast for schemas / types)
• Mule connector does require this (probably as any other ESB)
• Mobile SDK so require this
• Studio would need that to build the target platform

We already have :

• a widdget / layout endpoint that was initially done for Studio and Android SDK
• a [schema/doc endpoint](https://github.com/tiry/nuxeo-doctypes-io) was added for Mule Connector

Globally what we would like is a consistent API :

    /nuxeo/api/v1/config/types

    /nuxeo/api/v1/config/widgets

    /nuxeo/api/v1/config/layouts

In addition of the pure REST binding, we probably want :

• a simple WebEngine UI
• an addon in explorer ?