More afterbars

Finally found a bit of time to work on this some more :)

I think we're going to change the command name to "trigger" to keep it consistent with the existing track-trigger stuff. I'm also thinking that keyword options will be:

     Trigger AT=xx   CMD.. /// do at specified bar
     Trigger After=xx  CMD.. /// to after xx bars
     Trigger Repeat=xx  CMD... /// do every xx bars

Multiple trigger lines are permitted, and each can have its own settings.

I don't think that every user will be using this ... but it's a simple addition and should make using melodies set in macros a bit easier.