Just a quick heads up as I saw that this is still relevant 
Last week I updated the code for the controls to the updated control scheme of ArcRotateCameraPointersInput.
It is much easier to understand now and I think the implementation is better.
It replaces the old ArcRotateCameraPointersInput completely so the following code is needed instead of just adding the input on top:
camera.inputs.removeByType('ArcRotateCameraPointersInput');
camera.inputs.add(new ArcRotateCameraSlideInput());
Aside from that it does exactly the same
The new code can be found in the gist from before: Babylon Arc Rotate Camera Slide Input · GitHub