Added (?) command.

Description: use (?) to show all commands available. Note that (?) is an alias
for (n).

Example:

    Score manager - example scores

        1: Blue Example Score (2013)
        2: Green Example Score (2013)
        3: Red Example Score (2013)

        scores - new (new)
        scores - show all (ssl)
        scores - show active (ssv)
        scores - show examples (ssx)
        scores - show mothballed (ssmb)
        scores - show user (ssu)

    > ?

    Score manager - example scores - all commands

        library - manage materials (lmm)
        library - manage stylesheets (lmy)

        scores - new (new)
        scores - show all (ssl)
        scores - show active (ssv)
        scores - show examples (ssx)
        scores - show mothballed (ssmb)
        scores - show user (ssu)

        cache - view (cv)
        cache - write (cw)

        LilyPond - view log (lvl)

        Python - doctest (pyd)
        Python - interact (pyi)
        Python - test (pyt)

        repository - add (radd)
        repository - commit (rci)
        repository - status (rst)
        repository - update (rup)

        scores - tour next (stn)
        scores - tour prev (stp)

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

        system - back (b)
        system - all (n)
        system - home (h)
        system - quit (q)
        system - score (s)
        system - session (o)

    > q