Collections in workspace configuration and applications

[ Feature description ]

* The workspace configuration describes the collection this
  workspace uses, and the kind of documents it contains.

* The workspace configuration describes also for each application
  it uses the collections this application has access to, and the
  role of the collection for this application.

* When the application is initialized, the collections are
  automatically loaded, using the storage engine class defined
  in the general configuration.

[ Core libraries changes - Applications ]

* New method Controller::initialize(), called by Controller::run()
  after the context has been passed to the Controller class, and
  before to call Controller::handleRequest().

* ApplicationConfiguration class has a new property $useCollections,
  an array to list the collections to use and their role

* Application class has a new property $collections, an array to
  the collections themselves, filled by the new initialize() method.

* Controller and Application classes are now abstract.

[ Core libraries changes - Workspaces ]

* WorkspaceConfiguration class has a new property $collections,
  the keys being the collection names, the values the collections
  document types to use. It's use by Application::initialize (cf. supra)
  to load the collections, and is also intended in the future to allow
  to browse collections from a workspace in a workspace settings web page
  or to print the collections not used by any specific application.

* WorkspaceConfiguration::getCollectionNameWithPrefix($workspace, $name)
  returns the full name of a collection, with the workspace prefix.

[ Core libraries changes - Collections ]

* New factory static method: Collection::load($id, $documentType)
  to get a collection of the type specified in the configuration.
  For example if $Config['DocumentStorage']['Type'] is 'MongoDB',
  it returns an instance of MongoDBCollection.

[ Minor changes ]

* Improved code documentation for Application class.