Already the oscilloscope calculated its length in beats, but
everywhere the variable was called "lengthInRows." Renamed the
variable to lengthInBeats and also changed the tooltip to be correct
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
