Updated three stylesheet file extensions.

OLD:

    experimental/tools/scoremanagertools/stylesheets/clean-letter-14.ly
    experimental/tools/scoremanagertools/stylesheets/clean-letter-16.ly
    experimental/tools/scoremanagertools/stylesheets/rhythm-letter-16.ly

NEW:

    experimental/tools/scoremanagertools/stylesheets/clean-letter-14.ily
    experimental/tools/scoremanagertools/stylesheets/clean-letter-16.ily
    experimental/tools/scoremanagertools/stylesheets/rhythm-letter-16.ily

NOTE: all stylesheet file names must end in .ily instead of .ly. This is not
yet enforced by the score manager. But this will eventually be enforced by the
score manager.