Add support for delegating navigation bar

Currently the package handles the navigation appbar and sometimes the apps' appbar is different making them inconsistent; it would be better to have optional delegation of navigation

Start by locating where the package handles the navigation appbar and how the navigation entry point is exposed. Clarify how an app can optionally delegate navigation to its own appbar, then verify that delegated and package-managed appbars remain consistent.

Written by the indexing model from the issue text.