Refactor parameter update broadcaster and listener

[jd-13]

[jd-13]

Partially completed in aea7a6e

The CoreProcessorEditor and CoreAudioProcessor now inherit from ParameterUpdateHandler and override its _onParameterUpdate() method, but they still need to manually register themselves with the ParameterBroadcaster owned by CoreAudioProcessor.

Would be preferable if this could be refactored so that all the registering and unregistering could be handled inside ParameterUpdateHandler.