Alto Trombone Timeline Updates

Many of the entries from the Alto in Treatises page had not yet been included in the Alto Trombone Timeline, so those were added to the timeline. They include many primary sources relevant to alto trombone history, including those by Seyfried/Albrechtsberger, Prout, Barrett, Claus, Schroeder, Vincent, Elson, Ergo, Clappé, Forsyth, Gilson, and White.