style(tracker): group code into less number of files

tracker/model.go

@@ -59,6 +59,16 @@ type (
 		modelMessages  chan<- interface{}
 	}
 
+	// Describes a note triggered either a track or an instrument
+	// If Go had union or Either types, this would be it, but in absence
+	// those, this uses a boolean to define if the instrument is defined or the track
+	NoteID struct {
+		IsInstr bool
+		Instr   int
+		Track   int
+		Note    byte
+	}
+
 	ModelPlayingChangedMessage struct {
 		bool
 	}
@@ -1474,6 +1484,14 @@ func (m *Model) computePatternUseCounts() {
 	}
 }
 
+func NoteIDInstr(instr int, note byte) NoteID {
+	return NoteID{IsInstr: true, Instr: instr, Note: note}
+}
+
+func NoteIDTrack(track int, note byte) NoteID {
+	return NoteID{IsInstr: false, Track: track, Note: note}
+}
+
 func clamp(a, min, max int) int {
 	if a < min {
 		return min