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
ec222bd67d