Add a parameter to force the display of the hidden documents in the document tree

.. even if the user preferences hides them. Basically we need a way to overwrite the user preferences for a specific document tree instance. The use case I have in mind is this: a technical page (which is hidden) may want to display a tree with nested pages that are also technical and thus also hidden. E.g.: for an application created with App Within Minutes the home page of the Code space may want to display a tree with the nested code pages.