ContextBrowserView: fix inserting nav wigdet into layout out-of-bounds

fe5b750d moved actions buttons which were in the main layout as first item out of it, placing them instead into the tool view's tool bar. While doing that, the commit missed to adapt the reinsertion of the navigation widget, which used to be the second item in the layout, to now be placed as first layout item.

In Qt 6(.7 at least) this insertion outside of existing indexes even triggers a runtime crash.