Add option to disable navigation collape when using root element

[angu-voy]

We wanted to have a page on an actual group level. It appears there is a root property you can use. The issue with it is that it seems that using it alters the collape behaviour of other parts of the tree. You cannot have expanded menu trees within the root level.

I would love a way to simply disabling this behaviour when using the root property.

            "pages": [
              "overview1",
              {
                "group": "v4",
                "root": "overview2",
                "pages": [