New extensible service as file manager for ExternalEdit

This pertain to the "Creation of a new Document through web service" use case.

When the user has finished editing a new document, she closes the client application and gets a popup dialog that asks here where to save the document in the nuxeo repository tree.

The list of options is to be provided by the WS server-side API to the ExternalEdit client. The list should default to the list of Workspaces the user currently has the "AddChildren" permission. The WS server should dynamically compute that list according to an extensible service (i.e. overridable by a extension point) so that customer project can register a custom Java class that is responsible to implement the custom business logics in case the list of workspaces is not enough.

The option selected by default should also be defined on the server side and overridable by the same extension point.