Add navigation sidenav

The navigation sidenav implements a part of the old "Pages" functionality. It is implemented as another sidenav, which is available to the user in both view and edit mode. Opening and closing the sidenav should work like the Components sidenav, and no more than one sidenav should be open at any moment, i.e. opening one may trigger the closing of the other.

The sidenav lists all "pages", i.e. all named (non wildcard) sitemap items of the current channel. Clicking on an entry has the effect that the CM navigates to that page, staying in the current view/edit mode. The sidenav shall stay open when navigating to a new page.