feat(tracker): oscilloscope and LUFS / true peak detection

In addition to the oscilloscope and loudness/peak detections, this
commit refactors all the channels between components (i.e.
ModelMessages and PlayerMessages) etc. into a new class Broker. This
was done because now we have one more goroutine running: a Detector,
where the loudness / true peak detection is done in another thread.
The different threads/components are only aware of the Broker and
communicate through it. Currently, it's just a collection of
channels, so it's many-to-one communication, but in the future,
we could change Broker to have many-to-one-to-many communication.

Related to #61

tracker/gioui/tracker.go

@@ -116,8 +116,8 @@ func (t *Tracker) Main() {
 	var ops op.Ops
 	for {
 		select {
-		case e := <-t.PlayerMessages:
-			t.ProcessPlayerMessage(e)
+		case e := <-t.Broker().ToModel:
+			t.ProcessMsg(e)
 			w.Invalidate()
 		case e := <-events:
 			switch e := e.(type) {
@@ -166,6 +166,7 @@ func (t *Tracker) Main() {
 	w.Perform(system.ActionClose)
 	t.SaveRecovery()
 	t.quitWG.Done()
+	t.Broker().Close()
 }
 
 func eventLoop(w *app.Window, events chan<- event.Event, acks <-chan struct{}) {