TrackList emits more events for certain changes of state of tracks...

... Also rewrote some of the existing event handling so all events from
TrackList are of the same, custom event class, and distinguishing addition
from resizing events, though this distinction is not yet used

src/NoteTrack.cpp

@@ -643,6 +643,14 @@ void NoteTrack::InsertSilence(double t, double len)
    // AddToDuration( len );
 }
 
+void NoteTrack::SetVelocity(float velocity)
+{
+   if (mVelocity != velocity) {
+      mVelocity = velocity;
+      Notify();
+   }
+}
+
 // Call this function to manipulate the underlying sequence data. This is
 // NOT the function that handles horizontal dragging.
 bool NoteTrack::Shift(double t) // t is always seconds