DJ Controllers: User-assignable controls

[madmaxoft]

Allow the user to make a mapping of their own between the controller keys, LEDs, sliders and wheels., and the UI handlers in the player.

The UI should register a set of events that it can handle, such as "start playing", "set volume", "move filter-list selection" etc., and the DJC setup UI would allow the user to map any key / slider / wheel to the events.

For the LEDs, the UI should register a set of statuses it can report, and the DJC UI needs to let the user detect all LEDs on the controller and then assign them to the statuses, possibly with a modifier (blink, invert, ...?)

[madmaxoft]

For the sliders and wheels, the direction needs to be reversible.