InstrumentTrack: manage MIDI note recording in NotePlayHandle

We must not record notes when receiving external MidiNoteOff events
as e.g. the sustain pedal still might be pressed. State tracking for
features like these is done inside NotePlayHandle so move the recording-
related signal emission from InstrumentTrack to NotePlayHandle.

Closes #168.

include/InstrumentTrack.h

@@ -204,8 +204,8 @@ public:
 signals:
 	void instrumentChanged();
 	void newNote();
-	void noteOn( const note & _n );
-	void noteOff( const note & _n );
+	void midiNoteOn( const note& );
+	void midiNoteOff( const note& );
 	void nameChanged();