Added (nav) command.

Function: (nav) command displays a multisection navigation menu.

Example:

    > nav

    Score manager - active scores - navigation commands

        back - go (b)
        home - go (h)
        score - go (s)

        commands - hidden (hidden)
        commands - navigation (nav)

        LilyPond - view log (lvl)

        Python - interact (pyi)

        scores - tour next (next)
        scores - tour prev (prev)

        source code - edit (sce)
        source code - location (scl)
        source code - track (sct)

        system - quit (q)

    >