Initial work on allowing audio items to output to more than one track
Duplicate the various bits needed to make per-audio-item-sends viable
Add a dialog for setting track sends, albeit it's not working yet...
Close the dialog when the user presses OK
It turns out it actually did work before;  I was just doing it wrong.
This commit fixes a few minor bugs in the new code.
Add the option to send an audio item to the sidechain input of a track.
Fix regressions that caused crashes when glue'ing audio items.