From Pokechu22: Rename gain to velocity on note tracks

"Velocity" is the technical term, and it's the term that's used everywhere.
"Gain" was probably only used because "gain" is used on wave tracks; since
note tracks aren't wave tracks (and shouldn't be treated as them) the
corresponding variables should be named velocity.

src/AudioIO.cpp

@@ -3790,7 +3790,7 @@ void AudioIO::OutputEvent()
          data1 = mNextEvent->get_pitch();
          if (mNextIsNoteOn) {
             data2 = mNextEvent->get_loud(); // get velocity
-            int offset = mNextEventTrack->GetGain();
+            int offset = mNextEventTrack->GetVelocity();
             data2 += offset; // offset comes from per-track slider
             // clip velocity to insure a legal note-on value
             data2 = (data2 < 0 ? 1 : (data2 > 127 ? 127 : data2));