AGOS: Give each MIDI track its own loop flag (bug #6665)

This is one possible fix for bug #6665 ("SIMON1: Music in floppy does not loop after sound-effect") that I've been sitting on for far too long. I've finally taken the time to play through the floppy version of Simon the Sorcerer 1 with it, and I didn't notice any obvious music glitches. However, I haven't played through any of the other games with it.

(Looking at the diff, I see I removed a comment with a reference to another bug when calling setLoop(). That's probably - as I said, it's been a while - because setLoop() now sets the _loopTrackDefault flag, so it wouldn't actually do any good to call it any more.)