The old mechanism made it difficult to follow exactly what happens
when a button was clicked, because the Action/Bool that gets
executed / toggled was declared ages ago, in the constructor. In the
new mechanism, the Action / Bool is bound to the button at the last
minute, right before Layout. ActionButton, ToggleButton,
ActionIconButton and ToggleIconButton were done to avoid heap
escapes: if the corresponding functions woudl've returned
layout.Widget, a heap allocation would've been needed.
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
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