Consider invalid song path given via command line a fatal error.

Avoid scanning notes_on for ports on which there are no notes playing.

Since there doesn't appear to be anything in the JACK API to either
examine the current timebase master, or inform the master that it has
been relieved, it will still require some timing hackery in order to
display "slave" when another client forcefully takes over as timebase
master.

that resulted in all non-note events being dropped.

Add commands to invert selection.
Make middle click toggle note selection instead of set it.