Add an Automation operation to publish a snapshotable folder

The nuxeo-tree-snapshot addon contributes a new factory, FolderishProxyFactory, for the publication which plugs very well into the Publication mechanism of the JSF UI.

However nothing is exposed for the Automation part, but the operation CreateTreeSnapshot. So the request is to add a new operation to pubish a snapshotable folder by doing the same as the factory FolderishProxyFactory.