Ctlra Mapping: Refactor to generic payload

[harryhaaren]

This tracks remapping the callbacks to contain a generic "payload" for the application. This enables different types of tracking based on each application: eg: Mixxx uses a ControlProxy to handle the value change - store a pointer (or the thing itself?) in the generic payload that is returned to the callback as void*. Cast to "payload_t" that was inserted, and any application can retrieve any data it requires.