Nomenclature change: 'invisible trees' to 'private trees'

'Invisible' is too like 'hidden' which is settable by the user (plus pages in these trees could be hidden too...). 'Private' more correctly describes the nature of these alternate content trees.
Ensure that private trees are rendered during publishing
Ensure that the File output store is applying the '#' prefix of private trees
Be explicit about which root we're asking/testing for: public or private
Ensure that the navigation helper works as you'd expect when rendering private pages
Allow image previews to take full width of panel
Correctly handle navigation requests from pages in private trees & don't throw exceptions if the template is assuming an incorrect page depth

Pages from private trees that are aliased into the public site can have templates that only render correctly when shown as an alias.