adds the possibilty to flip the value of the slider

I'm presently doing a template to listen music with the "auto dj" feature (to use Mixxx as a simple player): https://github.com/demos/developer_skins/tree/master/Advanced-Player.

To display the first track on top and the second one above, I needed a property to flip the value of the slider so I added a "Reverse" boolean element. 

I'm absolutely not sure that "Reverse" is a good name. Why not "Flip" or "FlipValue" or "ReverseValue"?...



Even if it's not important, I just want to notice that the two elements "Horizontal" and "Reverse" could be replaced by a "Direction" one having "left, "right", "up" and "down" as possible values.

Thanks in advance,
Jean